With some minor editorialising.
1975: The Russian mutant Colossus, Piotr Rasputin, joins the X-Men in America.
1978: Colossus enjoys some sex with two women of the Savage Land.
1980: Kitty joins the X-Men, introduced as age 13 ½. Colossus soon catches her eye, to his embarrassment.
1981: Kitty calls him “sexy” and steals a mistletoe kiss on the cheek. Aside: the word “sexy” should be illegal in the X-Men. Her crush is overt and acknowledged by their adult teammates.
1981: A potential future-timeline is established in which Kitty (by then Kate) and Piotr are married, and Kate has had children (deceased). This flash-forward story takes place in 2013, making married Kate Pryde a perfectly acceptable 46 to her husband’s fifty-ish. However, Kate’s presence in 1981 is designed to avert her terrible, dystopic future and is a successful mission, removing the necessity of an adult romance between these characters from the X-Men playbook.
1982: Kitty tells a bedtime fairytale to Piotr’s visiting little sister, in which the X-Men all take on exaggerated “ancient” orientalist counterparts. Kitty is a pirate and Colossus her first mate. This, within her story, develops into romance.
1982: Alien-impregnated and heading into the sun in a doomed spaceship, Kitty and Piotr share a kiss and talk obliquely about how if she were older, they could have sex before they die. But she isn’t—so they don’t. Colossus’ age is given, in the comics, as 18 during 1982: Kitty is about five years her beloved’s junior.
1983: Kitty turns 14 in a Special. The general rule of X-Men time passage is–or was at this time–two and a half print years to approximately one in-universe year. Kitty’s fourteenth birthday party taking place in a bonus issue may mean she turned 14 at this point in the print chronology, or it may fit into an earlier gap in publication. During this story, Piotr’s age is given as 19. Kitty states her intention to date Illyana’s brother.
1983: Fall-to-the-floor open-mouthed snogging and thoughts of marriage abound; Kitty pledges herself to a life of misery to save Piotr (she’s saved herself in turn) and Piotr gets needlessly jealous of Kitty’s same-age, same-intellect friend Doug.
1984: Piotr comes back from Secret Wars and dumps Kitty on a hill by telling her he fell in love with a beautiful spacewoman who understood his love of agriculture. She is broken-hearted. It’s a gorgeous, gorgeous, well-delivered tragedy—just great comics. But a bad, bad romance. Kitty is comforted by Piotr’s sister Illyana, her best friend.
1985: Kitty Pryde meets Rachel Summers, a traumatised fugitive from the future timeline Kate Pryde helped avert. Kate, who called Rachel “poppet” and “my love,” manipulated circumstances to save Rachel by sending her back in time. Kitty, who saw Kate’s time while Kate saw and manipulated hers, recognises and welcomes Rachel.
1986: Seeing his teammates grievously injured during a fight, Colossus flips out and murders a guy.
1988: Colossus is upset at the thought of his Savage Land lover’s possible murder. She survives, and introduces her son, whose father is “absent.” She asserts that she will raise him alone.
1988: The X-Men, including Colossus, sacrifice their lives on live television to defeat the Adversary. They are resurrected by magical ally Roma, in secret, and granted the ability to escape record and publicity.
1988: Kitty joins Excalibur, a British superhero team. Rachel Summers is also on this team. Piotr remains with the now Australia-based X-Men. As far as she knows, he is dead.
1989: Kitty develops an immediate crush on Alasdair Stuart, an adult ally of Excalibur.
1989: Piotr paints teammate Betsy Braddock in her bedroom. She is nude; he wears red shorts and—crucially—red boots.
1989: Kitty does her first murder.
1990: Amnesiac after several traumatic rebirths, Piotr (now Peter) lives as an artist in New York, dating a now-beautiful version of older woman, ex-X-Foe, and veteran fighter Callisto.
1990: Kitty sucks cake icing off the finger of (villain Sat-yr-9 disguised as) Courtney Ross, an older ally of Excalibur. It’s Kitty’s birthday; she’s turning 15 (the year to years ratio is out the window, I suppose). Courtney takes her to London and Paris, buys her jewellery and a dress, and lets her drink champagne before putting her on a boat with some men for the night. The next morning, they closely embrace, and Courtney gives Kitty a car.
1991: Piotr regains his memories and is obliged to rejoin the X-Men. Upset to lose his happiness, he travels to visit the Savage Land and learns “his son” is not his son.
1992: Having lost the physical beauty she’d enjoyed with him, Callisto dumps Peter for his evil brother and dies.
1992: Piotr’s parents are killed. His sister Illyana, once again a child, is brought to live with Colossus and the X-Men.
1993: Illyana catches ill and Kitty visits Illyana at the Xavier Institute. Illyana dies. During her funeral, Piotr abandons the X-Men to live in a mutant separatist state in space. Kitty returns to Excalibur.
1993: X-Men convince Kitty to act as “bait” for Colossus on Muir Island to make sure he joined Magneto’s Acolytes of his own free will, and not because of a previous head injury. Kitty and Colossus comfort each other over Illyana’s death and, determined to be mentally fit, Colossus goes back with the Acolytes. Kitty kisses him goodbye.
1994: Colossus has to be stopped from beating an enemy, Fitzroy, to death.
1995: Legion kills his father, Professor X, in the past, altering the timeline into the dystopian Age of Apocalypse. Colossus and Kitty are now married, but when Colossus finds out his sister Illyana must sacrifice herself to help restore the rightful timeline, he goes into a berserker rage and kills Kitty while she tries to talk him down.
1995: Kitty begins dating Pete Wisdom, an older ally of Excalibur. Both assume their age difference to be 10 years; assuming the 1:2.5 year rule and taking her introductory age as the baseline, Kitty is 18. Calculating from her textual fifteenth, she’s 17. If time moves so slowly that it takes 10 years for one character to age a year and a half, she’s 15… and a half.
1995: Returning from her apparent, but not-actual death, Callisto reconnects with Peter. She leaves again to teach some rebellious youths. They kiss, and she promises to be “back” for him.
1995: Immediately after his kiss with Callisto, Piotr visits Excalibur and is driven to such extreme jealousy by Kitty’s relationship with Wisdom that he tries to murder him.
1995: Colossus joins Excalibur. Kitty continues her relationship with Wisdom.
1996: Kitty awakens with Wisdom’s clothing strewn about her room. They have implicitly had sex.
1998: Colossus nurses a crush on Excalibur teammate Meggan, spending the wedding of Meggan and team-leader Brian Braddock (Captain Britain) scowling at the groom.
1998: Kitty’s pet dragon tries to reunite Kitty and Piotr.
1998: Kitty and Wisdom break up after Kitty wonders about their age difference and Wisdom indulges too deeply in his own sense of guilt.
1998: Excalibur disbands, and both Kitty and Colossus return to the X-Men.
2001: Colossus dies.
2001: Kitty visits Russia and scatters what she believes are Colossus’s ashes.
2002: Kitty attends university. She comes close to sharing a kiss with fellow Xavier alumnus Karma (a girl).
2003: Kitty rescues Rachel Summers from Elias Bogan, and they desperately embrace.
2004: Colossus is discovered, by Kitty, to not be dead. He is on a violent rampage, which she is able to stop.
2004: Kitty and Piotr pick up their relationship again, after twenty real-world years. If the 1:2 ½ formula is still accurate, this is an eight-year in-universe break, and makes Kitty 22 to Piotr’s 27. Rachel is jealous.
2006: Actually the Savage Land non-son was his son, after all. Piotr does not know.
2006: Kitty and Piotr have had sex.
2006: Kitty believes herself to be pregnant with Piotr’s child. This is a psychic projection, and not true at all.
2006: Kitty gives birth to their imaginary baby. She and Piotr name him Michael. But he’s not real: he is part of a villain’s mind-tricks plan.
2008: Kitty is believed dead.
2009: Colossus gets a tattoo of Kitty’s nickname and a large pink heart across his chest. His bereavement is also represented by an empathic mutant with tattoo powers, who manifests a drastically tacky tribute tattoo across his entire torso. It features Kitty’s face, a huge heart, several tableaux of Kitty and Colossus embracing, and a Star of David (for her) on his right shoulder, across from a hammer and sickle (for him) on his left.
2010: Kitty is discovered to be alive, but unable to touch or verbally communicate. She is kept in a tube. Kitty and Piotr converse via the psychic ability of team telepath Emma Frost.
2011: Kitty is once again tangible.
2011: Colossus gets in some violent catharsis whilst imbued with the spirit of breaking stuff. Kitty ends their relationship, noting Piotr indulges too much in his own sense of guilt.
2011: Kitty is pregnant?? No, it’s just an alien, again.
2012: Kitty goes on a date with Colossus, who plans to propose to her and is empowered by the Phoenix Force. He takes offence at her worries about his powers, they argue, and upon returning her to the school where she is headmistress he begins a fight with her staff which culminates in her attempt to murder him. He leaves. He feels sadly guilty.
2012: Kitty begins a short period of dating Bobby Drake, Iceman, a member of the original X-Men who had left the team (graduating from Xavier’s school) before Colossus joined it.
2013: Colossus and Domino, an older member of X-Force and veteran fighter, begin a relationship.
2014: Kitty begins a relationship with Peter Quill, aka Star-Lord.
2015: Kitty’s fear of space is put to one side after Star-Lord is captured (in space).
2015: Kitty and Peter (Quill) have sex.
2015: Star-Lord proposes marriage, and Kitty accepts.
2015: Colossus and Domino’s relationship receives its last on-page appearance, though their break-up is not shown or implied to be approaching. Domino states her acceptance that care of his sister Illyana will always come first, for Piotr.
2015: Kitty does her first murder in space.
2015: Kitty breaks off her engagement to Star-Lord.
2016: Chris Claremont, co-creator of Kitty Pryde and Rachel Summers, co-creative lead on X-Men titles for a solid sixteen years (1975-1991) plus later runs, obliquely reveals his own headcanon endgame romance for Kitty: she marries Rachel and becomes the American President. They have kids.
2017: Kitty Pride leads the X-Men.
2017: Kitty discovers Piotr had moved into her old bedroom, during her time abroad with Star-Lord.
2017: Colossus tells Kitty that, should they date again, they should date with intention of marriage. She asks for time to think.
2017: She has thunk. Kitty surprises Piotr with a proposal, beginning their official romance yet again.
2018: Kitty and Piotr get married… unless, pray Chris, it’s all a hideous fakeout. Stay tuned, true believers.