It's part of the main story. You just have to say yes to sex, it's not hard.
Yes, we technically already have in the form of possession and corruption. The former involving 2 people, 1 unconscious and one fully conscious, the second involving an aphrodisiac or drunk like suggestion on the mind which is borderline mind control although unintentional. Alice is also considered a lover, so it's only a matter of time before she is willing to do something sexual without the influence of mind control. Violet we technically already have, she also has lover status, but she wasn't in her own body so it may not count as full sex. Gloria was the ex lover so it's likely.
Ok seriously, the questions you ask, if you actually watched and read through the sex scenes, you would know the answer to the questions you are asking already.