Need help!! 😁

Okay, so I'm 17, my SO is turning 18 in August. Every time we have sex he always asks if he can cum in me.. And I want him to but I'm too afraid. I am not on BC because I'm still too shy to even talk to my mom about it. Lol. It's bad. It's just how I've always been. But he proposed November 21.. Keep in mind that we've been together for two years now. I'm just too afraid to let him and he always asks why when I say no and I always don't know what to say to him! I love him so much and I want a family with him.. Just not now..