I don't think it is something you can set without a mod; but more like a hidden skill where it is there but you don't see. My sim as the romantic trait and I notice if I leave her alone, she will flirt with almost anyone if she is feeling flirty (her husband, random sims, the grim reaper, etc...); but noticed she does flirt with male sims more than female sims but had flirted with both on her own.
As for the patch, I think they just tuned it so it would be more or less successful depending on the sims your sims flirted with. If they flirted with male sims, romance with them will be easier than female sims. Not impossible; but you probably have to go out of your way at that point to build up their relationship if you want them to have it instead of flirting on their own as the game at that point sees their preferences for males.
