I have done my own research on this since noticing strange relationships starting in my game. I basically ran the game with a save with no mods other than MCCC to make it easier to perform troubleshooting and to also keep track of my sims gender preferences. I've definitely noticed alot of swinging of gender preferences, closer to neutral percentages, than their was in the past. I believe this is a combination of a few game changes that may not be WAI.
It seems that the whims that sims get, especially on NPC's, are alot more friendly/romantic social than in the past, and it also seems that these interactions are either repeating so frequently, or they have such a strong influence, that they are shifting otherwise completely straight or completely gay sims over twords the other side, especially if they have alot of friends of both sexes.
Also I believe some of the traits having to do with friendly and romantic interactions are stronger now, making it even more likely any one sim with these will become more of a gregarious romantic than what might have been otherwise intended.
One of the things I tried for example was to remove any traits from some sims that appear to affect friendliness or romantic feelings, then run some testing again. It definitely appeared to reduce the instance of sims who were mostly one gender preference from becoming unbalanced away from the way they currently feel.
Obviously when it comes to relationships and gender preference in the game it is a very fluid system, allowing for sims to be played first one way, then the other without having to start a new sim. But it seems like the random influence on sims, especially NPC sims, has become a bit too excessive.
Anyway FWIW, YMMV :D