The team crash-landed on an alien planet at the very end of X-MEN GOLD #19. According to stuff that I've read, Colossus was severely injured in the crash and almost died. Kitty decided that she couldn't go on without him. As a result, she proposed to him once they returned to Earth and once he wasn't dying anymore.
And presumably he said yes, because they will get married in X-MEN GOLD #30 in June 2018. (Jon's Note: I can't figure out the math here. That's ten issues away and only six months from now. Maybe they're switching to a bi-weekly publishing cycle?)
Fans of Kitty Pryde might remember that she was recently engaged to Peter Quill (AKA Star-Lord of the Guardians of the Galaxy). They're obviously not together anymore.