Soba the Casanova or Not?

Don't the pictures below say it all? Soba is a pretty dominant dog. So, when he plays with more submissive dogs, the following type of actions will occur! I've been told that humping is a way to show he is dominating another dog.

We see a lot of this when he plays with his girlfriends - Roxie and Madison (both are at least 3X his size). He spent an afternoon with Roxie yesterday, and there was a lot of humping action. Roxie's mom calls this "doggie porn". Yikes! =)