Curiosity is normal. There is a difference between curiosity and lust. I think trying not to notice can make you actually think about it too much, and that is why you are feeling uncomfortable.
In all honesty, it sounds like your overly self-conscious, and insecure with your own body. Are you worried about size? If so, it means absolutely nothing. If you are I could convey a series of events that would indeed lead to sin, ie. you are worried about size because you are thinking of fornication, and are afraid the girl will either poke fun, tell others, or not stay with you, etc. etc.
As for the inital curiousity, I guarantee there are many people like yourself out there, with the same question/concern. What you do about it could result in sin, but as for the intial question, no it is not a sin.