I've posted before in another thread-- unless it's totally a pr0n story that I'm reading PURELY for gratuitous purposes, I dislike sex scenes in fiction or movies, especially when it's set up and predictable: "Ya, they're gonna DO IT now... there they go..." I've read only a handful of novels that included graphic sex scenes with the details spelled out that I enjoyed enough to keep reading past that point. The thing is, it's hard to be graphic like that and keep your narrative going; sex scenes in novels and movies are always kind of like an eddy in the flow of the story in my opinion.
It's not enough that it helps define the person's character-- you have to ask yourself whether it moves the story along or if it's more of a break in the narrative. Most often I would rather assess another person's character by seeing what kinds of books he reads or how he treats his dog, ya know. If the genitals come out at an inopportune moment, I tend to lose interest in his "character" and everything else except getting the hell out of there.
Sex is never and should not be a dignified process, but people should at least strive for dignity outside the boudoir, whether they achieve it or not. If a person needed to see me having sex to fully assess my character, I would distrust him or her... or at least until sufficient green had crossed my palm to create a bond of commerce, rather than emotion.
But if you can do it and keep your story interesting, then by all means GO FOR IT. The above remarks are just my opinion.
