"Men have external genitalia, while women have internal genitalia. This simple difference makes a lot of difference in how they write about themselves— and how you might write about your characters. Male writers don't often address internal sensation in a character, because they don't experience it (and probably often don't realize consciously that it's there). This accounts for a lot of Really Terrible sex scenes written by men (if you look at the "Bad Sex-Scene Awards" in any given year, you'll see that the vast majority are done by male writers)."
Sex
Writing