From what i've seen with my sims, well, they're pretty easy! Ok, it does depend a lot on the traits, I had one sim who had the flirty trait and she walked into a park, no hestation or deviation she just walked up to a male sim she had never met and they started kissing! I was like, wowa there girl, take it easy! It was funny though. However I think it's as has been said, it's really up to you. If you start a gay relationship they'll have more gay wishes, just like if they eat a certain dish from a restaurant they'll immediately want to cook that dish.