Is it normal for a guy to feel insecure about sex?

I know it's normal for girls to feel insecure. But as a man, I'm supposed to be confident, right?

A girl I like invited me over to her apartment. We ended up sitting on her bed watching movies. At around 2am, she starts stroking my thighs. I should be excited... but instead, here's what's really going through my mind:

- I have zero experience. She's going to be terribly disappointed.
- What if I'm the wrong size?
- I ate Taco Bell there times this week, and it shows.

In the end, I decided to pretend I was really, really interested in the movie. It ends, and I say goodnight and go home.

We used to be great friends, but now she's ignoring me. I see her in class, and she pretends as if I don't exist. She has even started openly flirting with other guys, which totally goes against her personality. I know she's trying to make me mad.

I don't know what to do or say, but I feel like the girl here. For what it's worth, I have a healthy sex drive. I think about girls all the time, and I even fantasize quite a bit (and that includes this girl).

But when it comes to trying to visualize myself actually getting naked with someone I like, I can't do it. I'm terrified.

Do other guys experience these insecurities? Do you just get over it? Or do I need therapy?