Some of us have a lengthy list of sexual partners of multiple genders; others have been only with people of one gender; and still others have never been in a sexual relationship. Just because we haven't been with people of more than one gender doesn't mean we're not really bisexual.
Being sexually open-minded doesn't mean we're down for everything
This point is particularly relevant, in my experience, to straight/cis men with sexual hangups. To get to the point… we're probably not going to be freaked out if you like butt stuff, and we're not going to shame you or call you gay.
That said, don't expect us to be down for all of your unfulfilled sexual fantasies either – just like lesbians, gay men, and any other group, we have boundaries, and some of us are just vanilla.