I don't do same-sex pregnancy for realism but I've never seen ugliness in adopted sims the way most people seem to, and I do allow alien pregnancy which is how San Su Hong and Blair Mamuyac got their daughter So Wen. Romance Sims are more likely to end up bi, as I find they are the most likely to be "curious". I usually use ACR. Theoretically a turn-on for one male and one female trait would indicate a bisexual, but it's difficult to do without a big complicated mod that's more than I wish to undertake. On the one hand, as a bisexual myself it annoys me to see monosexism triumph; on the other hand -I'd have to be fiddling with his settings constantly, if I don't want to install ACR, and I don't. If I'm reading it right, without any mods, sims all start of Bi unless they are greated already married? Homosexual women are mostly happier with their lives than heterosexual women.
Because the control has been given to the player in the creation of their Sims, EA is able to avoid stereotypical traits in how gay and lesbian characters are depicted, while not alienating potentially conservative gamers.
All teens and older Sims can go back into Create-A-Sim via testingcheats or simply "Change Sim" on a mirror, dresser or closet and customize their gender to be able to impregnate or be impregnated, so same-sex couples can have biological children. Pollux Stacks grew into a straight Romance sim because he brought girls home from school every single day and managed to magically avoid meeting any boys but his brother. In The Sims 3 and The Sims Medievalmarriage is available to any couple in a romantic relationship, and Sims of either gender will recognize their spouse as "husband" or "wife".
I usually decide orientation by the first sim they fall in love with.