Family issues???help dad is over protective!!!?

Ok my parent let us kids do what we want, we comehome when we want,we sate out for as long as we want, we have phones with internet, they let us go where we want when we want unless were grounded. But when I told my parents I had a boyfriend and they freaked out on me???? Im 15 and he's 17. My parents are four years apart!!! This is so stupid. Why is my dad saying I shouldn't date guys yet when he let's me do all these things??