I am addressing this to sweet but clueless young men who want to approach someone nicely for sex, which is nearly all of them. I’m gonna state my assumptions explicitly, because I’m tired of comments pointing out the obvious exceptions. For the purposes of this post, this is a situation between peers, more or less, with no large power differential. There is underlying respect and goodwill between the two, if not crushes or love. So here you go, Ben. To get between making eyes at someone in class and creating the possibility of sleeping with her, I want you to do three things.
I. Assume that she is an adult who likes sex and can handle herself.
B. Pay enough attention to her to judge whether she is interested in you.
3. Ask her explicitly if she wants a kiss, or touch, or sex.
Step A.
Trust her. Women aren’t any more fragile or delicate than men. They will not be damaged by a respectful request for sex. (They might be scared if they feel trapped into saying yes, but since you are far more likely to blurt out an abashed “Um, I like your hair” while staring at your feet, you shouldn’t worry about that.) Trust her enough to bear half the burden of arranging sex. Maybe, because of outdated mores or insufficient interest, she isn’t going to take the step of initiating, but in that case, decency requires that she do the other half, which is to accept or decline politely. Got it so far? You will not do her any harm by nicely asking her out, or nicely asking her for sex. Even more, because she likes sex, you are doing her a favor.
You guys know this, right? That women like sex. They want to have sex. If you are a sweet, gawky college boy, awkward college girls would like to have sex with you. I can’t tell you how many dorky college boys I had lurid fantasies about. Those boys probably never knew, but I would have freakin’ loved it if one of them had been friendly and forward, and made it easy for me to live out those fantasies. I had to kiss every one of my naïve boyfriends first; it would have been nice of them to make the first move.
Part 2.
Be interested in her. Get a sense of whether she is interested in you. Here are some ways to tell if she wants you bad: in a crowd, she listens for what you are saying; she laughs a lot when you talk; she sits next to you and doesn’t move away; she touches your arm; you find yourself alone with her. I’ve said this a couple other places, but it is worth saying again. People involuntarily face what they are interested in; their chests and shoulders will be square to the target of their attention no matter where they’ve turned their heads. When people like you, their bodies will mirror yours. Look to see whether you are both leaned forward, holding drinks alike, crossing arms or legs alike.
The part about finding yourself next to or alone with women is an important clue. Sweet young men, you may not know this, but women scheme. Sometimes it is coincidence that she shows up at the dining hall at the same time you do, but sometimes she went to a lot of trouble to learn your schedule and walk to class at the same time you do. If you are coming back from some group activity, and the two of you just happen to split off for somewhere? She arranged that shit and you never saw it. She did that because she likes you, and would like to have sex with you. Now do your part and put the moves on.
III.
The actual words aren’t as important as the attitude it is coming from. For casual sex, be friendly and respectful. Be easy with any choice she makes. It would be fun to have sex; it would be fun to make out for a while; it would be fun to do neither and keep talking. Do what you have to, so that you aren’t desperately wanting her to say yes. Know that you will have lots of other opportunities. If you would rather be more serious about her, be sincere and straightforward about your feelings, and still ready to hear either ‘yes’ or ‘no’.
The actual asking? Try to initiate some casual touch, like leaning on each other, letting your thighs touch when you sit next to each other, or letting your arm rest on her somehow. If she doesn’t move away, you’re in. Ogged’s recommendation, to ask to kiss her, is a very good one. It is both sweet and erotic to say “May I kiss you?”. If she already wants to, she’ll completely melt. If she doesn’t, she’ll say no, and you are easy with her choice. Once you are already kissing, you can always say things like “Do you want me to do this more?” or “Do you like that?” or “What do you want to do next?”. Those are entirely appropriate questions, regardless of how experienced the partners are. You can murmur them all naughty, if you like, and they will still work as a way to check in with her.
You can do this. You should do this, as a kindness to awkward college girls who are just as tied up in knots as you are. It will get easier as you practice. When you are a little older, both people will have learned some skills for exactly this situation, and that is easier still. You have no idea, gawky college boys, but you are adorable; someone is thinking impure thoughts about you right now. Easy and straight and respectful. Go.
(A lot of this is a synthesis of Unfogged comment threads I can't find now. Other smart people should take credit for much of the good stuff here.)