If you're playing a version before the old sex system was removed, as it sounds like you are, there is a sex tab on the slaves themselves. You go there. It starts with just kissing and massage I believe, but each action gives a little affection. After enough affection (the highest needed is 20-25) you can click on 'unlock sex' or whatever the button says, and use the affection points to unlock more advanced things. Petting (fingering/handjob/other hand related acts), basic sex (missionary/anal/other basic penetration acts), Bondage (self explanatory), etc.. It's not really needed, but it's one of the only ways to get the more, niche sex acts.
In the interaction mode, the most extreme you can do is whipping, really I believe, otherwise it's just fondling, kissing, fingering, etc. Nothing real extreme. It needs to be fleshed out, but it's not bad. It gets you lots of mana for almost no effort, doesn't take your energy, and can get foursomes going without having to unlock it with affection.
Basically, this sex mode is for generating tons of mana, while the other one is used mainly for intimacy purposes, say, for when you like a girl(or guy/futa, your preference xP) and want to do specific things. Not that any of them are particularly descriptive.
Anyway, long answer short, If you're playing a version from before it was removed, there's a tab for sex on the slave page, looks like 2 hearts. If you're playing a version from after it was removed (patreon only at the moment I believe? I could be wrong) then they haven't removed the references to the system.