Kind of like saying a female human can't rape a male human, it's possible. I mean what would you think of an older lady who forces herself onto a 7 year old boy who has no clue what sex is, gets his dinger to get hard and goes to town? That'd be rape. The body reacts to physical stimuli, sometimes the reaction isn't what the mental part wants but it carries on anyway. Just because a dog get his red rocket out, doesn't mean he wants to fuck. I've seen dogs get a hard on from being so excited for playing fetch and simple petting. There's also a good chance that that dog got a taste and enjoys plowing human pussy.. They have their likes and dislikes just like us. Not all dogs love rolling in shit and stuff that smells bad, some are prissy and don't want to get wet or muddy. Some hate the snow, some love it. One experiment with one dog isn't enough to prove what all dogs like.
I think that's your dog's way of telling you he doesn't approve of the quality of women you are taking home.