Well I'm more going by classical Greek myth, which paints Hercules as quite strongly bisexual. Yes, he was married at least once or twice (to women), but his exploits with male lovers are also front and center in the myths. I'm aware that Marvel excised that part of his mythological characterization, but I'd see it returned to him if I had my way.
Also, there's the fact that he hails from a culture where homosexuality was accepted and respected, and I'm certain that he'd want to be a part of a group that demonstrated against modern homophobia. After all, the Hercules of myth would find such attitudes both baffling and infuriating, and completely alien to his cultural sensibilities.
Again, this is going by the actual classical myths, here, rather than strictly by the Marvel version of the character.