lol...there's a 'point of no return' in ME 3 where your relationship status becomes formally 'recognized' by the plot. It's not in the same place for every character, but somewhere along the lines anyone you're in a relationship with will ask you to confirm their status (this is not what happens early in the game with rekindling an old relationship). If you say yes, other relationships vanish. If you say no, you lose the potential for relationship with that person.
In my last playthrough, I had both Kaiden and Liara invite me to the Citadel at the same time. So I had to go reject Kaiden to his face...then walk over to a spot practically in VIEW of that spot, and tell Liara she was the one.
Little awkward. Easy to imagine Kaiden just like...watching me go over there. Kind of kills the mood.