How can I persuade my mom and dad to let me marry my boyfriend?

My mom and dad absolutely hate my boyfriend! I'm 13 and he is 17 and they think he is a loser because he dropped out of school, and they think I'm too young to be with him. They also don't like him because he smokes and drinks and they he's a bad influence even though I don't smoke or drink! We want to get married because I might be pregnant, but my parents don't know. Its not something we planned I just wasn't careful enough. My boyfriend says that if we can't get married he's going to leave me even if i do turn out to be pregnant. I don't want him to leave me and i don't know how to convince my parents that we are meant to be together. The only time I get to see him is when I sneak out and at school:( It makes me so angry when my parents call him a child molester and a predator:(