Different people have different bone structures and different body types. There's nothing weird about your gap, lots of people have it.
I've heard that a girls hips may get wider after she starts having sex- I have a hard time believing it though. For one thing, how can your body differentiate between sex and masturbation? In my opinion, it doesn't add up.
Whether or not sex sex affects your hips, puberty certainly does. That's probably all it is in your case. Your hips widen to make room in case you have a baby, and some people's hips widen more than other people's do.