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

It sounds like you already know why you're sad. You felt like you guys had a plan, and he went back on it. Don't be too hurt though, a lot can happen in the heat of the moment that isn't planned for. If you still want to wait for marriage, you should absolutely do that. I'm 25 and I can tell you it's the best way to go. You'll feel better about yourself and your future wife will appreciate it I promise.