I've seen Dickens's little female friend Callie hump him like there's no tomorrow. There are other aspects of behavior involved, I think, apart from the sexual (and hormonal) ones.
Look at it this way: as humans we understand sex on a rational, thinking level. Dogs don't have this attribute, so nature has to program them to get the job done without really understanding what it is they're doing, or why. It's a complicated thing! It of course involves a lot of instinct, which relies on certain things that trigger the instinct. And some of the instincts evidently aren't limited by the sex of the dog, or the presence or absence of hormones.