First of all, I haven't played a demo for The Sims 4 and have been playing The Sims 3 unfortunately only up to Generations, so maybe this wa already gone through somehow but I don't know.
Where the request comes from is really funny. I had played The Sims 1 and The Sims 2 with all expansion packs, and The Sims 3 up to Generations, and have seen that in every single game and expansion pack, sims are "plain bisexual". The gender difference brings only female sims to give birth (or motherhood basically) and breast, there are no behaviour differences between genders, and a male sim desires to be with a female sim (and vice versa), unless he or she is requested to do otherwise. This is where the "plain bisexual" sense kicks in, sims feel like they can make love to anyone. So, as a request (and I hope I'm not too late) I was gonna suggest some deeper behaviour (and maybe ability) differences between genders; as in men, for example can be better in sports and women better in cooking, or a men greeting each other can be different than the way women doing so - it'll be upto your creativity.
The request kinda goes even deeper here, to eradicate the plain bisexuality, actually being able to make a sim gay, lesbian, bisexual or straight, most probably in the Create-A-Sim screen or as they turn into young adults (or teenagers if you can). I mean, I like having my homosexual sims but you end up forcing them to realise, until then a sim is straight but as you give a command for a homosexual act, he says why not. If we could decide a sim's sexual identity, that'd add some different traits into the game, the easiet to foresee would be a girl rejecting a guy's kiss because she is a lesbian, disables a male sim directly to hit on men because he is straight and so on . This might also please the LGBT players in a way.
Let's say the sexual identity couldn't make it to the game; yet further than giving birth and such natural abilities, I still believe sims should have exact behaviors regarding the fact that they're male or female. I hope you can do something about this in the new game.
Thank you.