Yea I can’t believe I’m asking this question either, I can’t believe I’m even thinking about it.
But as pretty much everyone with dog experience knows dog sex is based around pheromones, male dogs only respond sexually to a female in heat.
I know there is the old joke about a dog humping someone’s leg, but I’ve never actually seen that in real life.
So yea…
I think you either have a very sheltered idea of canine sexuality, or a view much more informed and nuanced than my own. I’m not sure which. Maybe both.
Some dogs will dry hump random things, although I grant it’s not really the same process as how they mate.
I have heard of dogs buggering other dogs (not in heat nor even female) but on reflection I don’t really suppose that involved the whole, “knot and stand there for an hour,” mating process.
I’ve seen alpha dogs, even female, do a sort of mounting thing but this seems more about submission than sex. The males when doing this are not erect, and it isn’t so much humping as just a still hold with growling.
I haven’t met more than a few handfuls of dogs in my life, and yet I’ve twice seen dogs that would move from object to object humping away. Legs, blankets, baskets, handbags, you name it. Not a “sort of mounting” thing either.
From what I’ve seen, male dogs have no problem getting an erection from physical stimulus, without a female dog in attendance.
Hell, my neutered dog gets erections when he’s overly excited from play.(ETA: great. Now if my wife searches my username, this thread title is going to come up. 
)
My neutered dog humps his teddy bear. He gets right into and even has a little canine erection.