It's a human behavourial question, so I think its appropriate.
Why do we noisily stop our dogs (or other pets) from having sex with other dogs in front of us even when they are neutered? I did this the other day again with Sam (my dog) who really fancies his (neutered) girlfrend, a very attractive poodle. I thought about it and I really should let them continue.
I think I would be most annoyed if someone did the same to me. So bloody rude!

