It's also a bad analogy: a dog reacting aggressively to their own image vs a human getting aroused by images of someone else. It's apples and oranges folks! I came up with two much better analogies

1. Ever think a dog is dumb for humping a stuffed animal? Human men can become aroused looking at flat images of nude women in black and white. <- Comparing dog vs. human arousal to external stimulus

2. Ever think a dog is dumb for growling at his reflection in the mirror? Human men can become aroused by looking at themselves in the mirror. <- Comparing dog vs (unusual) human reaction to seeing their own reflection.