You know how we can control whether or not they can impregnate or be impregnated? And fashion choice, male/female, pee standing or no? I think that menu needs to be expanded.
Prefers Male/Female/Both
And two more traits, Faithful and Frisky. Faithful sims, once they have a romantic interest with someone, will never develop a romantic interest in anyone else, unless forced by the player (you). Frisky sims will flirt with anyone regardless of romantic status.
Those two things would fix almost all these problems.