Oh, Colossus's weird obsession and awfulness to Kitty has been well established outside of any time she had a new beau on the scene.

Such as his obliviousness to breaking her heart when he met someone during Secret Wars and didn't consider her feelings at all and then went and got into a bar fight with Juggernaut because he threw a tantrum when Nightcrawler and Wolverine called him out on it.

Or blaming Kitty for his not being there when Illyana died of the Legacy Virus.

Or the horrifyingly creepy way he howled Kitty's name over and over again while getting her name tattooed across his chest.

Just off the top of my head. I know there's more, including one in the new X-men issue this week, but I don't want to think about it too much.