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.