OP wanted to have a threesome with a bi girl and unlike many people who want this and even have a willing girlfriend, they had takers. OPGF vetoed that and picked a lesbian. She already knew she didn't want him to have anyone else. Now they are kids and maybe she was sort of putting it off an hoping he would forget or something, I dunno. But unless OP is lying about something there were pretty clear signs from her actions, whether caused by subconscious or not, that something was going to go wrong.

In what way did this not have to end in a break up? This was clearly always going to end up in a break up as soon as GF changed her mind, assuming that she did, certain actions she took suggest she was never planning on keeping to the agreement, even if she was in denial about it and wasn't willfully lying. Given their ages I'm pretty willing to believe she was just in denial.