Herpegonasyphillaids
Though I like the idea of the parasite laying it's eggs in him.
Herpegonasyphillaids
Or what if he woke up the next morning in the woman's body?
On the other hand, if you want to actually curtail the amorous activities, you may try this approach that's worked for me when I've needed to behavior modify a player:
Discuss the situation with the player, and how the character's womanizing antics are disruptive to the game. Explain that you understand that it's "Just being in character!" Then 'together' come up with an adventure plotline that will 'mature' the character - in fact you have a few ideas already (courtesy of this thread)! Something monumental happens, agreeable to the player, that will permanently alter his character.
I realize that discussing plots with players is unusual, but when your goal is to alter a character's basic traits, you need buy-in ahead of time.
It's pretty tough to set something up that only targets the PC in question, since the party should be a team after all.
I do really like the idea of making him pregnant though...
But then that has issues of it's own.
Perhaps a polymorph effect (courtesy a mage/exarch/deity) triggered by the act makes him become a her and pregnant? Of course, it depends just how far your players are willing to go (especially the guys).
If you can find it, the Book of Erotic Fantasy has a wealth of material--vampire or ghost (in a magical shirt that works like ghost phase weapon in reverse), diseases, a local culture that equates pregnancy with marriage and enforces its cultural norms (obviously a Lawful bunch), dopplegangers, deities, other outsiders (succubus/incubus).