Having trained my four dogs I do not have this issue. I believe they are not copulating but, trying to establish dominance. I'm not sure that particular behavior is proof that they will hump anything. My friend had a female poodle who used to hump furry toys and she wasn't rubbing her parts on it. I think she was small and felt like a boss in doing it.
What happens when they aren't trained against? How far would they go? I claim the untreated and unneutered males would actually ejaculate.
Do people in these situations let dogs do this as long the dogs would prefer is the real question.
Note that you just discarded a honest sex act of a dog as just "rubbing genitals"? So when is a sex act sex act and when it's "just rubbing"? Is licking genitals, what dogs do, a sexual act? Was Clinton actually right, what he had with Monica was not even sexual?
Actually "many" animals aren't choosy, having no problem jumping on the same sex or even corpses or non-living objects, and don't make long-term unbreakable bonds. There are enough links here in responses.
Honestly, have you never had a male dog trying to copulate with your leg?