I know so many people who have humped dogs I don't even think anything of it, anymore. That's probably not a good thing...
Whether or not it's OK to hump a dog, or let a dog in heat hump you or whatever I don't know... If the dog actually wants it, is it OK? Does that count as consent?
Honestly, it's not my thing so I don't really care and I haven't found a good enough answer to these questions to actually form a solid opinion on the matter. I've heard it from both sides of the fence (usually in a pretty heated debate) and nothing solid has ever come of it.