I very seldom see same-sex relationships emerge spontaneously from non-directed Sims
@Sindocat - This reminds me! I was playing the 2nd or 3rd HH I'd made myself in The Sims, soon after Living Large came out. The HH consisted of two female roommates, Kianga and Pilar. I left them alone in their LR for a few minutes while I was doing something else, and when I came back they were flirting and romancing each other! I hadn't planned on that, but of course I rolled with it. I don't remember now whether we could officially marry same-gender sims, but I certainly considered them married. They didn't cheat on each other, either, as another of my should-have-been-monogamous-sims did in that game. I was happy when--in TS2 or 3--they gave us the ability to make a sim monogamous if we wanted to. My sims come in all varieties, & I'd like them to act in character even when I've rotated to another HH & am not controlling them atm.
eta: agreed, in TS4 I haven't seen this happen spontaneously--but I sure saw it in TS1!