If this were true, I could never have any friends at all, being bi. I think the idea is just silly. I have straight male friends, straight female friends, gay male friends, gay female friends, bi female friends … (no bi male friends at the moment, but that’s just a matter of happenstance). It’s all fine.
I don’t get the notion that sexual attraction, or the possibility of sexual attraction, means two people can’t be friends. Five years ago, I had a serious thing for my best friend, but I’m not in her preferred demographic and it was obvious nothing was going to happen between us. So … nothing happened between us. I continued to value her as a person and a friend, didn’t do anything to make it weird, in time I got over it and our friendship is still going strong today. And when it’s just a matter or attraction, not deeper feelings, it’s even simpler! Some of my friends are pretty, so what? Why would that be such a huge issue that we can’t be friends?
It’s entirely possible to find someone sexually attractive* and do absolutely nothing about that attraction*. I find most women who aren’t at the extremes of either weight or age at least a little sexually attractive (I’m a lot pickier about other genders). That includes all but three of my many female friends. That doesn’t mean I can’t be just friends with them, or constantly think of them in a sexual way.
Learn to look at feelings of sexual attraction as aesthetic judgements not action imperatives (i.e. don’t be led by your dick) and you’ll find getting along with women (and gay men) a lot easier.