I've created, tested and balanced a fully functional sex simulation tool for different RPG systems. It is highly based on D&D mechanics but fits almost anywhere with minor adjustments. You basically add 2 extra pages to the character sheet and you're good to go in both fantasy, sci-fi, post-apo and any other imagineable systems.
It has its own sexual levelling system (separate from the normal one), utilizes both standard stats and special, sexual stats invented by me. You unlock sexual positions, choose them while having sex, inflict pleasure "damage" in turns, gain different feats, skills, develop them etc.
What you will not find here: no rape, no sex with animals, no sex with minors. I can't forcefully prevent you from implementing anything on your own in private games but please, just don't be degenerates nor psychopaths. Sex with monsters, demons, mythical creatues and tentacles, as well as some hentai-based not realistic sexual actions, which have been implemented in my system, seem already enough and I am not planning to change that in any future.
PS: Xenophilia is considered an attraction towards other races, species etc. so we may consider it a xenophilia if any inter-racial fantasy/sci-fi sex occurs.
1. PDF file with rules, required tables, modificators and mechanics descriptions. It's not as complex as it seems but basically allows anything you'd ever want. If not - feel free to modify but please - again - don't by psychopaths. It's already quite hardcore but if your mind goes further than mine - it's not my business :-P
2. GIMP2 .cfx file template with many, many layers to just click/unclick and choose different characteristics for your heroes/heroines/NPCs. 300dpi A4 format if anyone interested in printing. WARNING: It may be a bit hardware consuming... for me eats up 9 out of 64GB RAM on a nvme SSD and still needs a couple of seconds to load while freezing occasionally on i7 11th gen Intel CPU... That's what happens with 300+ layers, I believe.
3. Example character at lvl 1 with most default, starting stats as well as a developed version so you may imagine a progression curve. I've tested all different progression states, it works as intended and it is balanced but here as well - feel free to change anything up to your liking. It may be a bother though - I used some statistics and algorithms from work to balance it all so good luck with changing many things and balancing them again :-P