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.
Having a gap is perfectly normal and, I'm pretty sure, isn't something is created or affected by having sex. And if it makes you feel better I've got a gap too; about an inch and a half, maybe two inches. :]
Who cares? If your boyfriend loves it then what's the problem? He's the only one you should be trying to impress. I mean who cares what other guys think, their not your boyfriend. Don't worry about it hun, I have a gap between my thighs too and none of my exes have cared about it. I'm sure it's not even that noticeable. Hope I helped:)