Let's look at this from a purely theological point of view. I say, if you want to, and they seem to be up for it, have a crack at them. Here's my reasoning:
1. There are some quite liberal Christians who don't consider sex before marriage to be a sin (though I'll grant that this is rare, especially in America).
2. If it's entirely consensual (make sure that it is), then you can't be accused of putting her soul in jeopardy. If she's prepared to have sex with you, then she's already condemned her own spirit (if that's what she believes)!
3. If she is a good Christian who always sticks to her beliefs of what is moral, then she won't agree to have sex with you.
In short, consensual sex lays none of the blame at your feet. You can hardly be accused of leading a girl astray - the only girls who are led astray are the ones that want to be led astray. Hence: go for it, and see what you can get.
However, this argument only works on the philosophical level. However true you or I might believe it to be, her parents/religious leader/whoever might not quite grasp it, and repercussions may arise! Make sure you've got a good get away plan, if you see what I mean.