The fact that I'm shaking already by purely thinking of a good enough of answer should show that I'm really afraid of it.
What fedcom said about being naked. Being naked makes me afraid. Vulnerable. Exposed. Insecure.
There's that.
Also I'm afraid of flashbacks. As some of you might already know from the confessions session thread: I was sexually abused over weeks when I was younger and also almost raped on a playground.I was around 5/6 back then.
When you have sex, you are completely naked. Not just in the literal sense. There's no acting out of it. Who you really are will come to light. How you moan, how you move, what you say, what you want from the other person. You are naked, not just your body. Your whole being.
Sex is something where you completely lose yourself, in a good way. Where you should be completely open. There comes a point, where you just grave for more while you're at it and where you lose all control.
Also I'm afraid of doing something wrong or not good enough

Or just being plane boring. Or moaning too loud
.. And I can't write much more about it as I'd have to reveal stuff (not just about myself) that I really don't want to reveal.