I'm a female and my best friend is a male. We're both virgins, he's 17 and I'm 16. He keeps telling me that he wants to have sex, and he's been touching my body more and more. We've kissed and made out recently, but he wants to go all the way. He tells me things like "I want to be your first, and I want to be your first." He says that he thinks sex will "strengthen" our best friend relationship but I'm afraid it might ruin it. What should I do?