An interesting idea for a story--but the theme goes against the grain of erotica. Being rendered less potent...hmmmm. I would only touch something like this if a stud gained unearthly powers, abused them--and some other guy, preferably inexperienced or with a score to settle, wound up with all the inches and cum-loads.
There is a Yahoo group about the stealing of male essence, but I
haven't seen anything sterling there.
I would like to see one anyway, if it's really good.