See also: How a Nerd Became a Dating Columnist
Unless Kinsey is back from the dead and observing us through a two-way mirror, don't refer to sex as coitus. Additionally, don't use childish euphemisms. It's creepy. Just ask comedian Patton Oswalt (the above video is totally NSFW, by the way).
How do you make first-time sex with a new person even more awkward? You say nothing.
I think of it as a moment of silence for something that died: our chemistry.
8. "I love you."
This was actually said during the act, making it only a million times worse. Your heartfelt proclamations lose credibility when you're sticking it inside someone. It's hard to feel you when I can still feel you.