How about this: the Mother is divorced from the Son's father, because he was a scumbag who treated women like dirt. She is afraid that her son will turn out just like his father, in fact she has already seen hints of this personality trait beginning to emerge. She wants to nip it in the bud.
She has a friend who is a bit of a mystic or a witch doctor or a voodoo practitioner or whatever. This friend arranges for the body switch. The Mother's goal is for the Son to live for several days as a woman, so that he can emphasize with them more fully. Of course, to fully understand what it is like to be a woman, that will have to include having sex as a woman. She can't expect her Son (in her body!) to go out and have sex with some stranger, so she has to take care of it herself.
Of course, in the course of the saucy five-page sex scene, they BOTH learn a little about the opposite sex. And about themselves.
Who knows? Maybe they both like the new arrangement, and decide not to drink the undo-potion?