Two of my sims are suddenly gay and in a relationship with each other

I always assumed there was some sort of algorithm in place for that. The sims, other than outward appearance, seem rather gender neutral. I'm thinking they are programmed with the "If not x then y." argument in mind. The game checks for gender preference (meaning previous romantic leanings) and if none is present then it does a random roll on whether to initiate romance in that current situation? I could be completely wrong, and it might not be that at all, but its what makes sense to me.