I once read somewhere that the things that they are most insecure about with their partners are the things that they are insecure with themselves (aka Takes one to know one). Even when I'm in a relationship, I do tend to have at least a little bit of attraction to my female friends.
I think (hope) it's natural. I mean, they're of the opposite sex and I like them, otherwise I wouldn't be friends with them. This isn't to say that I want to jump all my female friends, but rather that there's obviously compatibility there. Also, if one of those friends happens to be attractive, well, you can't really just "turn off" the noticing gear in your head
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Assuming that I'm somewhat normal feeling like this, it's no wonder guys can get insecure about this. It's got to freak them the hell out that a) they know other guys think the same things, and that either b) the girlfriend is completely oblivious to this, or c) the girlfriend is quite aware of this and is actually attracted to them but keeps it on the down low.
I try my best not to worry about it though, because no matter what there's nothing I can do to control the situation. The best I can do is try to not worry about it, because that's just the way things are.