What should I do?

I love my boyfriend, I see myself marrying him, having a family, growing old together.

The only problem is I feel like 19 is too young to be completely committed to a single person without exploring. I'm bisexual, but have never been able to really do anything with my sexuality and I don't know how to bring this up to my boyfriend.

I don't want to hurt him, he told my friend that he wanted to propose next summer, which terrifies me.

How do I bring any of this up to him? Should I even bring it up? I feel like I'm lying to him and myself by not saying anything. I need advice.