The implications of sex are MORE than just the risk of pregnancy. Sexual intercourse is an ADULT activity. It's something that grown-up people, in long-term, secure, loving relationships do to help enhance that relationship through the giving of pleasure to each other.
At 13 you are NOT an adult. You are a young adolescent. You've likely been with your boyfriend for what ... a few months? That may feel like a long time at 13 -- but it is not. You may think yourself 'mature' -- but in your case, being 'mature' means the ability to recognize what you are and are not ready to handle. And that means WAITING to have sex until you are an adult. There are many other ways you can spend time with your boyfriend.
(And consider: Are you willing to tell your parents that you are having sex? If not, why? If you are ashamed of it, why are you doing it? If you think THEY would be upset, why would you think it's ok?
And consider -- no birth control is 100% effective, and teens tend to be less responsible about using it. So if you WERE to get pregnant -- what then? Have you talked with your boyfriend about what you would do? With your parents? If not, why not?)
There is no rush. You will have scores of years as an adult to have sex. You only have a few years to be a young teen.