Having a sim do romantic interactions with another sim affects that sim's gender preference. If a sim flirts with a sim of the opposite sex, 2 points are added to the heterosexual score and 1 is subtracted from the homosexual score, with the reverse occurring for romantic interactions with same-sex sims. So, if sim Suzy has 10 romantic interactions with women, she will have a homosexual score of 20 and a heterosexual score of -10. Once you've built up some points on the homosexual meter, Suzy should only autonomously flirt with girls. If you do enough of both, you can end up with a bisexual sim, but you have to balance it very carefully if you don't use mods.
**Please note, I don't know the names of the scoring meters, so I used "score" and "meter" here with "heterosexual" and "homosexual" as parameters. If anyone knows the real names would you please let me know? Thanks!