My Chis hump one another all the time. It's obviously part of their play or one suddenly gets horney. See....gotta tell you a secret. (don't tell anyone) They love each other sooooooo much, you just wouldn't believe it. There is never a time when one is trying to one-up the other about anything. One way they play is that they'll race through the livingroom, into the kitchen being all silly, play vocalizations etc. Then they'll come to a stop and Chuli will get this funny coy look to her...where she turns her head just a little, freezes and makes tiny, tiny, tiny little minute jerky movements/hops. One little hop and she'll do it again...and again, turn 1/4 turn and another little hop about 1/4 inch off the ground and nip playfully at Jose`'s neck, wagging her tail. Then race around in a circle, engaging her hind legs way under her. (a blur) Then she'll start humping him or he will her. It looks more like she's flirting than anything. Then off they go again playing tag. It's a scream to watch their antics. But there is never a competition thing with them about anything at all. They get along better than just about any dogs I've ever seen. They just like to hump each other.
More recent stuff I've read indicates that it probably isn't frequently a dominance thing, if at all. They think it's more about sex than formerly thought (the M word) and also used in play.
Sex or humping IS a primary reinforcer just like food. So, that explains why Jean Donaldson let a dog do this. It is a viable and valuable reinforcer. It's embarrassing to us because according to our culture, that is gross looking. But who are we to say what is okay for dogs? In using that as a reinforcer, it doesn't mean that the dog is going to hump whenever he pleases or take over Jean's world. Because Jean knows behavior, I'm sure she puts it on cue and so the dog only does this upon invitation.
There are a lot of myths about dominance that need to be thrown out with the garbage once and for all.