The whole experience, from the ritualization of sex to the substitution of a male "teacher" in lieu of a female is, as you put it "on the weird side of the meter". (I love that phrase, I'm going to have to remember it). As Tibial said, the official explanation behind it is so men can better understand the "pain of penetration" and therefore, help men become "better lovers" by identifying with and having a better idea of how to please their female partners. But with so many things, there is a deeper level behind this ritual, one of the possibilities being that this is another way for a female to emphasize her power in a matriarchal society, by forcing men to interact with each other in a sexual way before granting rights to sexual union with a female. In cases like this one, however, it can backfire.