I agree with Kevin. C**k and p***y are not good words to use, and the same can be said for penis and vagina. Note: While I am gay, I am not above reading about straight sex so long as the man is the main focus.
Figurative language is the best. It makes the flow more artistic, and it's certainly more appealing than vulgar or clinical words. I can't read much of anything on Nifty, because it's just too damn vulgar. It seems like many on there just write about meaningless casual sex. What happened to making love? There are some stories on GA that I love because of the author really brings you there and makes the scene come to life. It's something wonderful being shared by two people. In one case, there were three, but it was written in such away that it still had a similar charm.