prom night

So my prom is in 2 weeks and my boyfriend and I have been planning the night for months! We're so excited and plan to have sex that we've both consented to. The plans are for me to spend the night at his house. The only thing is my family is hella nosey and always feels the need to know about my sexual life. I'm almost 18 and I feel like I have the right to keep my sexual life to myself, but they feel as if I must tell them everything. Anyways, I'm going to lie to my mom and tell her I'm staying at a friends house. Does anyone have any advice or something to make me feel better?