Once. In this one RP I sorta got roped into smut stuff. I sorta half assed it on my part, but I can see why some people do it. I don't see  explaining how the one part diddled the other part is important to the plot though. I think a simple fade to black really does the trick better. Less awkward for those around you, streamlined and fluid with the writing, and sometimes can come off rather comical if you do it that way.

However I do believe there is place for sex and sensual behaviors in character/ romance development. Passion can be a very strong reason for character motives during a story.

Of course though if fiddling with diddles and doo-das is really your fancy you should still be technically able to live that out in PMs.