These lesbian friends most likely identify themselves as feminists, but I don’t think any real feminist would think they can judge another person’s sexuality by how they look or act. I bet if you tried to tell them you thought they were straight because <insert hetero girly thing they do> they’d rip your lungs out. By the same token, they aren’t allowed to judge your sexuality at all. The only thing that makes you gay is if you have sex with and/or date men. Until they see you actually having sex with a man they basically can go fuck off. And being lesbian doesn’t make them experts in all things gay, and definitely doesn’t mean they know anything about men any more than you know a whole lot about lesbians because you’re a straight man.
You also are not “feminine for a guy”. Being a human means you’ll have different behaviors, and taking out the damn trash or being polite doesn’t mean you secretly have an extra X chromosome or blow dudes on the sly. Anyone who thinks that knows absolutely jack shit about men, and trying to categorize you into “alpha” or “beta” is exactly as bad as you trying to say your girlfriend is a “slut” or a “good girl”. I’m positive if you told her lesbian friends that you thought they were sluts they’d probably kick your ass. But, they can go around saying you’re gay? Nope. If they don’t want to be judged for how they present their gender then they can’t judge you.
And more importantly, why would three lesbians care if you’re bi-sexual? You have sex with your girlfriend and love her? So if they think you’re gay then that’d make you most likely bisexual by their standards. They are basically telling your girlfriend that because you might be bisexual she should run away and not love you? They might as well be saying she shouldn’t love you because you’re short, or Asian, or talk with a lisp, or have dyslexia. They most likely believe sexuality is core to one’s being, so if your girlfriend loved you she’d accept any attraction to men you (do not) have and move on. Now, I’m not saying you are or are not gay, but I’m saying if they think that you are, then they’re violating quite a few of their own beliefs.
My advice is you sit with your girlfriend, and these idiots, in the same room, and you flat out tell them that it is incredibly un-feminist to judge another person by how they act or look; that if you said that you thought they were actually straight they’d lose it; that accusing you of being gay when they’ve never seen you with a man is wrong; and that telling your girlfriend this is interfering with your relationship and that they’re going to stop right now. Don’t let them debate it. You flat out tell them you’re not gay, and they’re being hypocrites if they think you taking out the trash means you have sex with men. If they try to say being a lesbian means they have some mythical gaydar then tell them nobody is psychic and no, they are not fag whisperers. They can’t read minds, so they can shut the hell up.
If they pull the whole “me thinks thou dost protest too much” because you’re angry they said you’re gay, just tell them you have a right to not be judged for how you present your gender. They would be angry if anyone tried to classify them as sluts or said being lesbian was just a “phase” and that they’ll “snap out of it”, told them what to wear, or judged them as straight for wearing a dress. Right? Well then you can be angry too. You are allowed to be totally straight and also dress anyway you want, be polite, take out the trash, and any other damn thing you want. Them saying you being angry is proof is just another way they try to judge you.
Then, if your girlfriend doesn’t shut up about it you should leave her. In fact, if your girlfriend doesn’t stop talking to these obvious saboteurs for a while then you should consider leaving her. I know that’s harsh but she clearly isn’t interested in getting to know the real you–a multifaceted man who is polite and caring–and is more interested in listening to her stupid friends. It also means she’s a hypocrite because she’s a homophobe who hangs out with lesbians. There are plenty of women who would not think this about you and also would tell their friends to shut up, so being 25 you have plenty of time to find that girl.
Good luck man. This situation sucks, but it’s mostly because people these days have an incredibly distorted view of masculinity that places us into “alpha” and “beta” categories. It’s not right, and you’re allowed to not be defined this way, and anyone trying to put you in those categories is wrong. You’re also allowed to be angry about it, and being angry isn’t a sign you’re gay or crazy. As I said earlier, if a woman can get angry about being called a “slut”, you can get angry at being called a “fag”.