How do i tell my mom?

I’m 18 years old, I’ve been dating a guy for almost 8 months. My mom has met the guy once and briefly. We will be moving to USA shortly and my boyfriend will arrive later. We will be in different cities but we were definitely planning on visiting each other. When i told my mom about this she became very protective of me, she is against this idea and she told me that she is going to lock me in a room if necessary. Later, I found out that she doesn’t want me having sex with him and that she’s scared. But truth is that I already had sex with him and we were 100% safe. How do I tell my mom that it is natural and that sooner or later this will happen? Please help me 🙏🏽🙏🏽🙏🏽