As there's a 0.3.3 early release on the patreon including sex system rework, I'll go through some changes for educational purpose.
Firstly, now you have to unlock sex options with a chosen slave. Without that you can't send them to brothel. Unlocking those depends on loyalty, slave's experience and affection (more on that later on). Sexual traits also make unlocking it easier, while being of same sex or close relatives will make it more difficult. You can also just announce that you'll be using them anyway without their consent (high penalty to loyalty/obedience and so on).
After that affection comes into play: every action generates some affection between you and slave. The more enjoyable action was, the bigger affection gain. With affection you get to unlock more actions. Many actions directly influence brothel jobs income and won't make brothel profitable without unlocking those (although having assigned to brothel also has some chance to unlock them).
Traits play certain roll in making many actions more enjoyable and also can be adopted by making slave orgasm from related action (guaranteed chance after casting Entrancement). Stimulating potion will increase lust gain from action. Many negatives won't really have enough effects right now and lust will grow much quicker than it should making things easier, so balance is not quite in there yet.
Anyway, I'll return to working on the alternative partners/group, text and polishment in few days.