Good eye - yep, that's the basic gameplay hook there. Sex in combat would be, in a general and abstract sense with probably a few different concrete avenues for it to manifest (general "looseness" factor, specific acquired perks/kinks, etc.), a short-term advantage sacrificing a long term one. You'd be more and more likely in future encounters/battles to end up in a situation where your character's willingness is more of a liability than an asset - and eventually that will checkmate you into a game over at, say, the hands of a sexy, sexy minotaur/giant/werebitch, or some enemy where having sex with them dramatically reduces your combat effectiveness (a succubus, for example), putting you at a disadvantage. It's entirely meant as a tactical vs. strategic choice.
So would this be tracked by a separate stat like 'sluttyness' or something to that effect? If so will one of the current base stats act as a counterweight? For example if a character has high charisma then will they be more able to engage in sex without as high a risk of consequence?
Oh and one further question if I may. For anyone planning on doing a purity or chaste route (or as close as you can do to one in this game), were their character tp receive some unplanned intimacy, (or even better, lots of unplanned intimacy) would the player find their chaste and virtuous hero corrupted into a sex addict? Because that would...'terrible'.
A character with higher charisma would have an easier time getting in to fight sexy times, or to defuse a fight with sexy times, or to avoid a fight (even without sexy times), but higher charisma itself wouldn't prevent the negative effects of sexy times, at least not directly. And yep, it'll be sluttiness, plus perks you build up (Level 1 Donglust, Level 2 Donglust, etc.) that might be unlocked depending on said sluttiness score.
Purity / chaste runs are actually going to be built in as challenges - ie tracked as an achievement or possibly even a mode. There would actually be a few different kinds of challenges - purity vs. chaste is a good example. As for that corruption scenario... yep.