Why don't you just give a wink to that girl and mindfuck her
kidding.
Perhaps it's just that you have been with your girlfriend for way too long and you seem to be settling down although you are young your hormones are flying and you are thinking that you are missing out since you settled down so early. Your goal at the moment is not to settle down and have a family but she is PERHAPS the right one but you just don't want it to happen this early.
I think it's a phase and as you mature you will realize that a lot more. As a friend I hope that you don't make the wrong choice for a fling of fun and lose your girlfriend. If that does happens I am sure it will be one of the worst mistakes that you will blame yourself for.
Well all that only counts if she IS the one.
Perhaps if you are in your mid to late 20's and settled down with a job and getting ready to buy a place you won't have this stuff on your mind since right now you don't have as much to worry about and your mind starts wandering.
But this is all just an assumption by me and I hope it helps
Cheers
If not we can grab a beer and discuss!