This isn't so much about Hercules being straight or bi or gay or whatever, but I think with Hercules many comic readers have a hard time understanding that Marvel's version isn't exactly the classic mythology character as detailed in old legends, as much as Greek mythology is incorporated into Marvel Comic's Hercules...it's still MARVEL'S VERSION of Hercules. It's not them ignoring all the myths, it's them taking their version and expressing him how they feel. No different from any Disney-fied character (Show White, Pinocchio, Cinderella, Robin Hood, Aladdin...even their own clearly heterosexual Hercules).
No version of Hercules takes every myth into account because those myths were fabricated by who knows how many people over however many years and often conflict. And in modern times...Kevin Sorbo's Hercules in the 90s, Disney Animated Hercules, The Rock's more realistic take on Hercules, Kellan Lutz's Hercules 3D, DC's all out villainous rapist version...there's so many versions of the character, Marvel's version certainly isn't the definitive version by any means.