Labrat and Quizzical Pussy both have up excellent posts about sex and adolescence, so I'm joining that bandwagon.
The thing that amazes me most about my memories of teenage sex was that I couldn't talk. I knew what I wanted, but I was terrible at putting it into words. "I really like you, we should spend time together" became "uh, hi... never mind," "I want to try something different in bed" became "uh, let's do... hee hee, stuff," and "I don't like that, please stop" became "oh baby, come for me now." (I was meek, but cunning.) But most often, all of the above became nothing more than "...". I could express myself a bit more physically, I had less trouble crawling all over a guy or pushing him away, but when I tried to use words I felt like shame and awkwardness were choking away my voice.
(Also, it took me a surprisingly long time to figure out that "harder" and "faster" were actual directions, rather than meaningless words of encouragement. Porn kinda led me astray there.)
I blame Ms. Conrad. She was my "health" teacher, which in high school mostly meant sex ed. She drilled into us, over and over, that sex happens because you don't have self-respect. If you have other reasons to like yourself, if you have accomplishments to take pride in and friends and family to do right by, you won't give in to sex. Girls really just want love and approval, you see, and evil boys use this to manipulate the weak ones into sex.
So imagine my horror at discovering that I wanted sex. I must be weak! In the game where girls win by not "giving in" to boys, I was playing to lose. What a sucker. It was bad enough just to have sexual desires, but talking about them would be flaunting my weakness. I was distressingly embarrassed to be horny.
I don't have a whole lot of shame issues these days, but as a teenager I was embarrassed by everything. I wouldn't take off my shirt in the locker room. I was furtive and red-faced when I bought underwear, gripped by the terror that if I chose anything too sexy the clerk would be "on to me"--would realize that I had sexual thoughts--and that would be terrible. Even as I made an erotica website with my friends, I never mentioned the possibility that any of this could turn me on. Porn could be interesting, or well-made, or funny, but the one thing I could never admit was that it was hot. Hey, I wasn't a loser. I respected myself.
Before long, my hormones and my curiosity--and, hey, my desire for love and approval too, Ms. Conrad wasn't all wrong--were far more powerful than shame. I took off my shirt in front of a boy and I let him see my (white, plain, high-waisted) underwear. But I just couldn't talk to him about what turned me on. That was going way too far. Having a libido was my secret shame.
I can't put this all on Ms. Conrad and her ilk. I was also just plain young, and confidence and communication didn't come to me at an early age. And my boyfriend wasn't much help, being a little too ready to take "um" for "yes" when it suited him. And I simply had no role models or social scripting for sex. The private nature of sex means that there's no learning by example, there's no picking up the way sex is "usually" done, but everyone (especially if they lose their virginity to another virgin, as I did) has to make it all up from scratch.
"Self-respect means saying no when you don't want sex" is true. But it's half the story. Self-respect means saying what you want, no matter what it is. It took me a long time to learn that.