Gay dogs

Your dog just might not be interested.  Some dogs are just hornier than others.  I don't think that domestic dogs necessarily have the mating instinct, meaning that they don't feel the need to continue their species.  They are taken care of by humans and if they aren't horny, they are not going to try to have sex with other dogs.  I haven't really heard of dogs being gay.  You didn't say that your dog has been humping male dogs, so I don't think there's much evidence for him being gay. 

I wasn't sure how to approach this question when I first read it. I mean, I have 3 dogs and I've never once thought about their sexuality. 

So here goes: I think all dogs sniff each other in the rear end to first identify their gender and maybe age. I wouldn't be surprised if maybe the smell gives an indication of friendliness too. So with that said, just because he's not humping every female dog, doesn't mean he's gay. It's also important to remember that dogs that are neutered/spayed are sometimes less inclined to hump other dogs/things because their sex drive is lower (and they're probably aware of their lack of ability to reproduce so they figure what's the point).