On a serious note, some health care professionals advocate men washing their todgers immediately post-coit to prevent the spread of thrush. Some women can apparently contract thrush at the drop of a hat (without sexual contact), and thrush is very virulent. So, because getting out of bed after a good bout of love making is no naff, I'm guessing the dunk was suggested.
I prefer the post coital lean-out-of-the-bed-but-don't-lean-too-far to find a bit of laundry
On the occasions I can be arsed (perhaps an unfortunate turn of phrase in this thread), This.