if a dog tried to mate with my girl i would make damn sure that he understood that he's not in the position to do that. making sure the dog understands its position is important, and for such an act a whack on its head is not too much.
are you dumb...?
why should my girl break up with me because i saved her from literally being raped by the dog...?
(also, just consider the strength of some of those big dogs while they **** . what are you going to do? stand there and yell at him until he came? pull on his leash while until he comes?)