I come closest to @RDC on the issue, though I think they and I somewhat disagree on the ideal societal nature of it, what with me being rather personally prudish on the matter.
If we're going to have a discussion about writing erotic fiction collaboratively, we need to accept that almost all societies view sexual activity as unique, and American society views it as especially private to the point of being taboo. Whether or not one agrees with that is not really all that relevant, if the discussion is how TS works in the context of larger interpersonal dynamics.
It also means that most analogies to other kinds of human behavior are going to fail, because they simply aren't going to be appropriately analogous. I understand that we have our characters murder people and we don't murder people. I understand that we have our characters do all sorts of things that we don't do. But I think we should admit that certain behaviors between humans can lead to strong feelings of intimacy (go sing a duet with someone. or dance with them. You're gonna feel something, even if you're waltzing the most chaste waltz ever waltzed), even when those behaviors aren't happening face to face.
Personally, I felt weird having my character engage in a romantic relationship with another character while I was dating someone. My girlfriend knew about it and was not opposed, so I continued--but I (and I believe she, though I never felt the need to ask) would have felt like I was cheating had I not done FTBs when things got steamy.
