Talking to mom about sex???

Lexi
I need advice! My mom is super chill with literally everything which is so weird because she's super weird about sex. I have been dating my boyfriend for 10 months and we are in madly in love. We are both almost 16 and we have already started having sex almost, but my mom doesn't know and I want to be open with her. I asked her a couple months ago if we could just hang in my room and she said no and got super weird about it. I would think that she knows we will have sex one way or the other and why not under her roof where we are safe? It just really furstrates me because I'm not as close to her and my sister is and I'm scared to talk to her about having sex. I feel like she will not let me and I don't know what to do! Please help!