In much the same way that the Sims don't have to deal with racism and such, I feel like they're not as restrictive (self or societal) as we real humans are in their preferences either. I like to believe they like or dislike another Sim based on who that Sim is, personality wise and with compatibility and all that, in a more genuine sense, rather than full of unwarrented biases and things out of the Sim's control.
In my mind, a Good trait Sim might not like a Hot-Headed Sim, because they have a tendency to be angry and yell at others. And not because they wear an ugly colored hat or they have a certain skin or something like that. How gender specific can Sims really be when aliens impregnate male Sims and no one bats an eye? :D