No sex drive

I'm turning 18 in about a week and I have no real desire to have sex. My boyfriend and I have never attempted before (both virgins) but I'm afraid that someday he's going to want to be sexual and I'm still going to be like this. Sex just sounds unappealing and I really can't imagine him fitting in me. I don't know if there's something wrong with me, if it's just him that I'm not sexually attracted to, if I'm just scared or if I'm actually only sexually attracted to girls (I'm bi). Is there anyone that's been this way before? What happened?