My closest, best friend is a guy, and I'm a girl. Let's call him Larry. Larry and I spend a lot of time together, and as such, we get asked if we're dating pretty often. However, this is not the case for two reasons:
1. I'm a lesbian.
2. Larry has a girlfriend, whom I'll call Sarah.
We explain those circumstances to anyone who asks, and that usually does the trick for most people. There's just one person who doesn't buy it and insists that Larry and I are an item: Sarah.
We've explained to her over and over that we're just friends and there has never been any romantic history between us, nor will there ever be. In spite of all this, she insists that I'm faking my homosexuality as a cover so that I can be with Larry. She even went so far as to read Larry's texts to make sure he and I weren't sexting.
I can understand her being worried, but I honestly think that her accusations are baseless, especially considering that we've told her over and over again that we are absolutely not interested in each other. My other friends suggested that Larry and I should stop hanging out, but neither of us wants to do that. On the other hand, Sarah is the first girl Larry's ever dated, and he really likes her. What should I do? Should I back off until they break up, or should Larry and I continue to hang out?
Well, first things first: technically, what you should do isn't really the question, here. This is Larry's issue to confront and resolve... with "issue" being the most charitable way to describe Sarah's behavior. (If not being charitable, one might use choicer phrasing... like, say, "batshizz shenanigans" or "pile of horse excrement.") Or in other words, your backing away from this friendship wouldn't change the central problem of Larry's girlfriend being a bugnuts jealous wackbag. If you weren't on the scene to trigger her suspicions, no doubt she'd just home in on and freak out about some other nonexistent "threat" to her relationship.
That said, you might as well examine your friendship, just once, to make sure it's not actually crossing any lines. (Don't worry, there's a reason for this; we'll get to it in a second.) Even when they're wholly uninterested in each other, close friends can sometimes unwittingly end up playing a pseudo-romantic role in each other's lives for lack of a better option—which can then cause strife when one of them gets into a relationship. So if, for instance, Larry is texting you constantly while he's out on dates, or if you guys do a lot of platonic cuddling, or if a typical hangout for the two of you involves trading naked backrubs and then gazing schmoopily into each others eyes while sipping one milkshake out of two straws, you should probably stop doing that stuff.
Because what you should not do, under any circumstances, is cease your relationship with Larry. Don't give credence to this girl's accusations by disappearing from his life, and for that matter, don't say that you "honestly think" they're baseless. You don't think they're baseless. They are baseless, fact, full stop. Your lack of romantic interest in him isn't a matter of opinion—and that makes your friends' suggestion that you placate the girlfriend and stop hanging out with Larry not just insulting, but insane. This person is so consumed by jealousy that she's monitoring her guy's text messages and accusing a lesbian of faking her sexual orientation; she is, at the very least, flirting with abusive tendencies, if not outright rolling around in them. (For a fun thought experiment, imagine for a moment how horrified we'd all be if this letter were written by a guy about his best girl friend's new boyfriend. Shudder.) And if you allow her to drive you away? That's how the SOs of controlling people end up isolated and vulnerable. It's bad, bad news.
Which means that Larry needs his close friends to stay that way, not just in spite of Sarah's paranoia, but because of it. So, stay close. Be unfailingly pleasant to her, and don't badmouth her to Larry; her behavior should only ever be under discussion in a way that encourages him to reach his own conclusions about it. (When she does something controlling, try asking him, "How do you feel about that?" or "Are you okay with that?"—and if asked for your opinion, stick with innocent regret: "I'm sorry she's not more trusting.")
Basically, keep being a friend to your friend—and continue doing it even if he makes the foolish mistake of distancing himself from you to please his girlfriend. Because when he does finally wise up and leave her, he's going to need someone close, caring, and supportive to help him make a clean break.