Why does my female staff sit on my male staff?

She does it all the time, he will be laying there perfectly quietly and she will stand up, walk over to him and just straddle him and sit down, it doesn't seem to matter where on the bod she sits, his head, his back, she will even settle down and lie right along his body if the mood takes her, he is incredibly tolerant of her, he is a staff x boxer so quite a bit bigger than she is but even so this is weird behaviour (is funny though) She is not the only female staff i have known to do this, my in-laws have a male and a female and she sits on him all the time, again, just walks over and plants her rear end somewhere on him, then, like mine, she sits up and looks around all pleased with herself, mine have been together for 2 years now and the other pair for just over that, what is the meaning of this strange but endearing behaviour?