The first time I say that, in the 1st scene with Asha, I thought it meant the MC was going to cum again and make her drink his cum. I never fell for it again. Really isn´t my thing.
I wouldn´t go that far, but it really is one I don´t understand. Same with feet, pregnancy and incest. But I´m still glad this game has all that and makes it optional for players, so that everyone can see their thing or ignore it.
It´s in the last update during the whole Ain sex scene. After you have sex with her in the common room, with Kateryna and Fairy (I think) watching, you can take Ain to the bedroom or the dungeon. If you choose the second option, which is the rougher option, you can have the MC do that to her.