It's bullshit that bodybuilding is a gay sport. It's bullshit that it's a sport too. In its competitive form, it's an original art form that exploits pushing the body to its ultimate condition. If some people eroticize that, it says more about them than bodybuilding.
As for its being mainstream, bodybuilding would have to give up too much of what it is to appeal to Mr. or Ms. Average. The best arbiter for a mainstream look is what sells in films, and the slim look remains in. If anyone beefs up for a role, he'd better ratchet it down for his next role or he'll quickly find himself typecast & limited in Hollywood.