Dear Ms Dog Gonnit,
Though homosexuality exists in the animal world (penguins!), I don’t think your dog is gay. Dogs that hump other dogs — male or female — may just want to assert their position in the pack ranking system; or just be excited.
And even if he was gay, you wouldn’t find a training class to undo it. Besides, it wouldn’t be the worst thing. For one, you wouldn’t have to worry about puppies! Secondly, having a gay best friend with four legs is best of all worlds!