It sucks because being called gay can really question or threaten your masculinity since, stereotypically, being gay and being masculine are mutually exclusive. Unless you happen to be homosexual and you embrace that as part of who you are, I don't think there's a good way to call anyone "gay." Make the gay equals happy remark all you want; it's still a very strong word of shame for some guys.
That said, you're dead on. It's easier to attack someone else than to be threatened by your own insecurity, and clearly the so-called friend is playing up on that. Why your other friends would join in on the fun is simply stupid. You know you're not gay. That's all that matters.
Is it worth it to try to talk to some of the other friends? Clearly, you may not be missing much with the fat guy, but if the other friends really hurt you and you think it's worth sharing that and maybe salvaging your friendship, I say go for it. On the other hand, if it isn't worth it, then maybe you're better off moving on. I imagine that you're a pretty nice guy who can make (nonjudgmental) friends easily, so you might just meet new people who are more fun to be around.