(Disclaimer: This is a naughty post. Not explicit but naughty. If, by any chance, you happen to be a minor, I advise you to not read it. Better click here. Or google words like porn, bouncy and boobs.)
RPGs and sex. They go together like bicycles and fish. Like unwashed teenage boys and 19th century architecture. Like the police and democracy.
Despite this, RPG sex is definitely lacking. The real problem seems to be a distinct lack of sex rules, that would seamlessly integrate the whole carnal experience into the game world. Rules that would intrigue the average Game or Dungeon Master. Rules covering such topics as Inter-Species Fertility, The Wild Thing or Sexual Psionic Powers. Rules that would be free.
Thankfully, another nonexistent problem has been solved. Just download The Complete Guide to Unlawful Carnal Knowledge (a.k.a. the AD&D Book of Sex), retrieve your trusted Advanced Dungeons and Dragons manuals and have a bath. A drink too. (Download link)
OR (and that’s an apparently big or) try the BBSW Roleplaying Game, widely recognized (by me and a few mates) as the premier Hentai RPG. Download it here. Oh, and in case you were wondering, this is a full-fledged stand-alone freeware RPG. And BBSW stands for Big Breasts Small Waist. Quite a shock, that.