There seems to be a rash of dominant behavior on the board lately. LOL! Humping is not a sex issue. It's a dominance issue, and the humper can be either male or female, and in any combination of male or female. Your female dog is asserting her place in the pack, and apparently your male dog doesn't mind being a rung lower than her on the pack ladder. As long as they aren't fighting to assert their position, it's going to happen. If you don't want it to happen, just break it up. Don't scream or yell or make a big deal of it, just use enough physical pressure to get the female dog's attention on you and away from the male.
I had the same situation myself, and my male dog would just stand there and grin idiotically while the female went to town on him. LOL Just know the behavior is entirely normal - for dogs anyway. :-)
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