So my fiction writing teacher's main thought on sex scenes was that they probably shouldn't be included unless they serve some specific function in advancing the story, or you can come up with some new way of doing it (no pun intended). Granting the fact that it would difficult to really think of something "new," as it were, I agree with his logic, but am also inclined to think (admittedly without much thought put into it) that if a story had an appropriate tone/atmosphere/ 'tude that a sex scene could be added as a kind of arabesque.

.............yes, that was a pun on my genes. And my jeans. As it were.