What am I doing wrong?

What are you doing wrong?

Maybe you're doing too much right.

What is it about you that's got these boys so excited?

Very easily, dump em or drop the date if that is the first thing they think of on the first date. Sadly around your age (and depending on where you live I suppose) your going to run into a LOT of guys at that age thinking with the wrong head. If you don't want to have sex, you need to make it clear and if they continue to pressure it, drop em. Not much else you can do because every guy is different, though if its a common thing, perhaps you should re-evaluate the type of guy you date.

Say it in a way that cannot be misinterpreted and if they don't listen simply brake up with them; their not the person for you.

Pick different guys and drop the ones who immediately bring it up. Don't bother arguing about it; they are icky guys. (Especially if you say no and they keep insisting. Date rapist in the making.) Look for a nice guy and if you find a toad, drop him. Maybe try hanging around with a different group. Most guys are not like this.

Be true to yourself-cliche but true. This is actually a type of peer pressure, like the guys who have commented, you shouldn't give in. If a guy starts talking about sex with you, just tell him upfront that you want more out of a relationship. If he's worth it he'll wait for you.