I'm thinking that maybe that those other dogs have been humping other females and the scent is still lingering on their male parts. When my BabyGirl is in heat I usually allowed her to have fun with my neutered male dog Louie (mixed breed) (not a good practice as I see she's pulled a muscle and is now isolated from that shenanigans). Well now my male bully Wilson is smelling Louies boy parts and has been trying to hump Louie. So perhaps that is why Opie is attracted to the other males. Just a thought...but I think since dogs are mostly scent-oriented I imagine it doesn't take much to get their hormones going despite what sex they are.