My best friend lost his virginity today, he's 15. Why am i sad?

It sounds to me like your disappointed in him. You know hes better than that for his words hes told you and he did a complete turn around and went against his "beliefs" or at least words. Talk to him about it. Say you overheard him talking and your concerned. He may have also been lying to fit in type of thing. I know many people whom have done this unfortunately.