Actually I've read that the scent of a human female's genitalia is similar enough to that of a female dog's that a male dog will want to have sex with her. And, I can't imagine that they understand that sex with a human won't result in offspring. They don't have a complex enough understanding of how it works. Also I have heard numerous accounts that once a dog mounts a human it will hump away all on its own, without coercion.
Actually I've read that the scent of a human female's genitalia is similar enough to that of a female dog's that a male dog will want to have sex with her. And, I can't imagine that they understand that sex with a human won't result in offspring. They don't have a complex enough understanding of how it works. Also I have heard numerous accounts that once a dog mounts a human it will hump away all on its own, without coercion.