I'm having a difficult time really cracking the problem. Easiest thing to do would be to add a stripping mechanic to the game (and I find the idea of Strip Chess hilarious).

It's easier to do with a video or computer game because it's a visual medium.

For board game that gave cards or points, one could replace the cards with some that have instructions for various acts, such as fondling another payer for 30 seconds, and getting more intense as the game progresses by swapping out for other decks at various points.

A tabletop RPG is trickier; no group I've played with has been close enough or secure and bold enough to include more than a "fade to black, time passes, fade back in" kind of sex scene. But there's a lot you can do there if you're running a game with your partner(s), especially in a setting that has magic. To a degree, you can do this to a limited degree with D&D and its derivatives without adding or changing anything; there's a spell in it called Mage Hand that's mainly used for moving light objects at a distance; but it's basically telekinesis with a limit of 5 pounds of force; it could be used for hands-free masturbation, groping or fondling at a distance, or manipulating items used as sex toys.

Another option is adding some steam to quest rewards; such as someone you rescue offering to bed you as part of the reward.

Adding things onto RPGs is where the real potential is; just add mundane sex toys as items for a nominal fee. Maybe some magic ones, like a butt plug that gives its wearer a charisma bonus. Maybe a bigger bonus (or only gives the bonus) if it's visible to others in-game. Maybe a bigger bonus if the player's wearing one too. And that's before using other spells in unintended ways; a web spell or animate rope spell save tons of bondage potential, a Summon Laborers Spell could be redone to summon potential lovers, and Unnatural Lust is already a spell in official 1st edition Pathfinder rules, although it doesn't last very long and was meant as a combat distraction.