Sexual orientation options?

That's almost correct. Every Sim in a fresh game has 0 gender preference, apart from some pre-mades who might have a slight inclination towards some gender. Even those Sims whose bio suggests otherwise start with a plain Zero. What determines their preference during the game then is the amount of successful romantic interactions which shifts them gradually towards one gender or the other. Just use a male Sim and have him ask every other male in town whether they are singles. If this interaction is successful, i.e., met with the approving reaction, this other male Sim will then be potentially gay, even if he is married. The further you progress in your game, however, the more difficult it will be to change a non-played Sim's preference, because they will interact autonomously with other Sims, too.
I have one Bridgeport game where Matthew Hamming was "accidentally" made gay by my own Sim right at the beginning of the game, because I used two romantic socials on him - Ask if Single and Ask Star Sign. After that, due to Hammings flirty trait, he must have turned many other male Sims autonomously because I ended up with half of Bridgeport being gay and most of the women being condemned to spinsterhood ;).