How do i tell my mum i want to have sex?

I am 16 and so is my boyfriend. we are both still virgins and we have been together for 2 years and 6 months. we want to have sex but we can't find anyplace or any time alone. i don't no weather to tell my mum or not, we have a close relationship but i don't want that to change. Also my mum and my boyfriend's mum are really close, he wants to tell his mum and the last thing i want is it getting back to her though my mum. what do i do ?? please help.. thanks