Look deeper. The reason bisexual females are more socially acceptable than bisexual males is because men are the ideal (due to religion, due to the fact that societies used to be hunter-gatherer cultures, whatever). Women who are masculine are "strong," while men who are feminine are "weak."
Keep in mind that I'm fully aware that not all gay men are effeminate and not all gay women are masculine and vice versa for straight people. It's the stereotype I'm talking about. And because of the fact that it's socially acceptable for a woman to act like a man but not the other way around, men who are effeminate stand out a LOT more. Words like "fairy" and "fag" come to mind about flamboyantly gay men, whereas I can't really think of any particular slurs about gay women.
I don't think the negative connotation is applied directly to bisexual men, only the flamboyantly bisexual men - which unfortunately is linked to the stereotype that all gay men are effeminate. If you want to be bisexual, go right ahead - but if you want to avoid the connotation, either do something about it and be one of those men who everybody is surprised to learn hits for both teams, or (my personal favorite option) just stop caring about the connotations one way or the other. So people call you a fairy. So what? It's just a word. Words can only hurt you if you let them. So your sexual status isn't socially acceptable. Who cares? You're not hurting anybody, as long as you're not one of those retarded P&P fucks. Does it really matter what everybody else thinks of you, if what you're doing makes you happy?
Once you realize the fact that people who are trying to hurt you are usually just doing they only thing they know how to do in order to cover up their own hurt, it gets a lot easier. The other people are just doing it from sheer blind ignorance - which, while frustrating, at least puts you in the cloak of safety of knowing that you're better than them and thus their opinions don't count either way.
Also, sorry if this comes off as slightly ranty. People who complain about things that are or are not socially acceptable annoy me. I mean, I understand that prejudice is a real issue and a huge problem in human society, but in most cases it's like somebody who gets all butt-hurt over somebody else's religion. Who the fuck cares what they think? The only thing that matters is what YOU think, as narcissistic as that might sound. The only time it should bother you is if you get fired or not hired due to prejudice, you know, when it endangers your livelihood. But even with that, there are almost always ways you can think positively about it. If you accept the fact that prejudice is part of the human condition and will NEVER go away (or at least not in our lifetimes), think of it like running into traffic. You can't really avoid it, it's going to happen sometimes. And if you get fired from a job because of prejudice, did you really want to be working there in the first place? Wouldn't it be in your best interests to find a different job where there is no such prejudice? And at least if you get fired and can prove it was for a prejudicial reason, you'll probably be getting quite a lot of money.
Anyway, that's why that annoys me. Feel free to disagree - I'll freely admit that I've never been discriminated against for something that wasn't just wrong - for example I've been discriminated against because the person thought I was gay when I'm not, but I've never been discriminated because of my race or gender. Which falls into the category of "plz don't talk about it when you don't know about it." *shrug*
