Only a small fraction of the entire audience is family and friends. You couldn't fill a whole hall with just family and friends. Also, the activity tends to isolate many bodybuilders so don't expect a lot of friends attending the shows and many might feel repulsed by the sight ripped bodies with veins all over.
It is mainly fans who pay the tickets to these events.
Truth is, not all bodybuilders are heterosexual. I dare say there is very balanced number of heterosexuality, homosexuality and bisexuality among them. Being heterosexual doesn't grant you any special right or monopoly on bodybuilding. Heterosexual bodybuilders and gay/bisexual bodybuidlers don't differ much from each other except for their sexual orientation. Likewise, heterosexual bodybuilding fans and gay/bisexual bodybuilding fans don't differ much from each other except for their sexual orientation.