My fiance's dog keeps humping a throw pillow

At this very moment, my son is "roughhousing" with my fiance's dog and the dog's favorite throw pillow. Thankfully, my son does not realize that the dog considers this more as foreplay rather than roughhousing. DS is laughing hysterically, running around with the corner of the pillow in his mouth while Simon (the dog) is chasing him with his thrusting pelvis and his creepy doggy erection (bleh). I know, I know...it's just a dog erection, perfectly natural, nothing creepy or sexual (in the human sense) about it. But, I hate this dog. I've tried so hard to love him and welcome him in the family as he's DS's favorite playmate, but I just can't make myself. It's been over a year, and I still cringe at the sight of the poor thing. He's got typical annoying behavorial issues (peeing on our dining room table would top the list), he's completely stupid and lacks any sense of normal canine loyalty to me or my fiance (he's reserved some for DS, but that's as far as it goes), he looks like a doggy version of a friend-of-a-friend whom I cannot stand, and he has really large, shiny, hairy, floppy dog balls that make me extremely uncomfortable. He reminds me of that nerdy guy in junior high who stares intently at your breasts with a distant smile on his face and an obvious boner...you don't feel exactly violated, just mildly intruded upon and irritated. I've tried directing DS's attention elsewhere to leave Simon and his pillow to their privacy until whatever needed to get done is done, but DS is having too much fun (and so is the dog, for that matter). This whole situation is making me nauseous...something about it all seems so inappropriate.