As a cis guy dating a trans girl, trans people (at least in my experience) really dislike when you treat being attracted to them as making you bisexual. Trans women are women and want to be seen as such, so if you treat being attracted to them as making you bisexual, that indicates you don’t see them as women. Genital preference is a thing separate from what gender you are attracted to. If a gay guy has sex with a trans man that does not make him bi just because a vagina is involved.
Actually, it’s called heterosexual because it has to do with sexual attraction, not with the subject of said attraction’s biological sex. That’s why there’s also the term heteroromantic, which has to do with romantic attraction. If you want to learn more, look up the Split Attraction Model (SAM).