I'm just hopping in here-
Writing, in my opinion, can have no boundaries. It's raw, uncensored emotion. There are so many taboo subjects that you can never really talk about in everyday life, that can be vividly described with little criticism in writing. Mibba is a place to express yourself, so if you feel it necessary to write about hot, kinky sex then go for it.
There is one current author that this makes me immediately think of, Ellen Hopkins. She writes beautiful stories with sex in them, and like I said before, taboo subjects such as incest. She is a pretty well known author in some genres.
And, adding onto the comments above, I wrote my first sex scene a few months ago. I deleted it five minutes later because it was shit.