It never ceases to entertain me how all of the geeky little 12 year olds (and many 20-30 yrold geeks, too) will jump at the chance to tell you to "bone her" or "hit that action" et al.... ad infinitum
Don't take advice from these people, as they've never had sex, and most of them have never even been to second base - or first base for that matter - with anyone but themselves.
Sex brings a lot of consequences, both physical and emotional.
First, physically you can screw up the rest of your life if you get a girl pregnant. At the very least you'll be paying child support for many years - If you try to talk her into an abortion, you can add baby killer to the list of what you see when you look in the mirror (unless you're a heartless asshole, in which case karma will have to teach you the hard way).
A lot of men will try to convince themselves that sex is only physical, but the ones who are able to convince themselves of this are either A) psycho/sociopaths, or B) complete losers who end up very lonely in their later lives, and die completely alone.
One of the few solid things I've learned in my life is to not take advice from people who don't know what the hell they're talking about because they've never been there.
Least of all from nerds who would freak out if a good looking girl gave them the time of day.
As for the topic at hand... most girls don't know what they want - being "second in line" doesn't mean ANYTHING. All it means is she isn't sure what she wants yet. If YOU actually like HER, I advise you to keep her in mind as a possibility, but don't devote yourself to her entirely. If a girl thinks she can have you too easily, she won't want you. Date around, and don't jump at any chance to interact with her. Make sure she knows you like her, but that you have other things going for you, and that she's not your "only hope".
Think about it - if she was obsessed with you from the start and wanted to spend every waking minute with you it would get freaky before long. There's a word for those sorts of people - they're called stalkers. Don't be one.