I had an interesting thought for a gameplay mechanic that could be played with to pair with my above idea. The sub has a 'consciousness' stat already and based around how much they can take. The dom could use a similar stat more loose like 'vigor' (basically energy). With this stat it'll show much much energy left the dom has and once the tanks empty they'll be unable to continue with their forceful actions. This either ends the interaction or if the sub wishes to take over and continue they can and satisfy their needs if the dom did not.
Obviously this stat could influenced by many things. The sexual deviant inmates could get a racial (haha) bonus like their stat is a maximum of 110% while everyone else is 100%. Agility stat could give a simple energy usage reduction to the acts in the interaction. The breeder and milking skills could have a perk or something to affect their actions, like breeder would reduce energy consumed breeding type actions, and milking would reduce energy consumed by breast actions(BDSM could have a similar effect on sex toys). Perhaps the more cum or milk a character has the less energy an orgasm consumes allowing the dom to go further.