In that case, you don't need any words, just good observation skill. When a girl wants just sex, she's gonna put some move, from trying to make an eye contact via licking her lips, dancing provocatively to behaving vulgarly. You can tell which girl is just after it by the way she's dressed and the way she's behaving.
If a man is just after sex then one of the best lines would be:
"Hi! I'm (name). Can we go somewhere dark? Just the two of us."
The line if full of language ambiguity.
1) Why would a girl think you want to do her? She'd probably think you're either lame and unoriginal or want her not to be single anymore, but be paired up with you.
2) And what if she is NOT single, but still wants to shag you?
I'm still gonna stick to my 1st post and conclude that there are no perfect pick up lines, and the best "line" is just what Mishanya typed "an intense, thought provoking and interesting conversation that goes for no less than two hours"