addicted to sex?!?!

I'm 16 and I've already slept with seven different men in one year (3 one nightstands, 3 which I met regularly) I was in a relationship with only one of them (for over a half year & I had my first sex with him). Sometimes I feel a little bit strange about it because of my age. It's like I can't say 'no' if a nice guy kisses me and especially my neck. any help how i can learn to say 'no'?