Even in 1984, the X-Men deciding to beat each other up was enough of a regular feature in the comic that they barely needed a reason to start punching, but this is a special case. It has its roots in Secret Wars, and while most people remember that as the crossover where Spider-Man got a new costume that wanted to f**k him so bad, it became the most popular villain of the '90s, that was far from the only notable event. The whole idea was that there would be a big change for every book, so everyone reading Marvel Comics would have to pick up Secret Wars in order to find out what the hell was actually going on. This is basically the formula that modern superhero comics are built on, which makes it the most diabolical and successful supervillain plot of all time.
For the X-Men, the shakeup came in the form of a wrecking ball named Zsaji, an alien from Battleworld who had the ability to heal someone from the brink of death, with the minor side effect of having the person she healed fall madly in love with her. After one particularly nasty battle, she worked her seductive healing on Colossus and then promptly died (drama!), leaving him to mope around Earth, pining for his lost space-love.
Which, of course, presented a slight problem when he got back to his regular girlfriend, Kitty Pryde, and decided to break the news with all the subtlety of a brick to the forehead:
Because if there's anyone you want to emotionally damage, it's a hormonal super-teen who can vanish you without a trace.