Nicy Zhu,
I was very surprised by most of the responses to your question!
I really disagree with most of them, I think they are naive and silly!
First, yes many people DO develop relationships from sex. I have two good friends who had sex on their first date. 9 years later they are still happy, living together, and they just had a baby this year.
The problems is not exactly about "sex first, or friends first".
Because sex can be a way to become better friends, and being friends can be a way to better sexual relations. For some people, sex is a way to know others, and they may feel that doing everything step by step is too business minded, not romantic, and too paranoid . They think romance must involve at least a little bit of risk.
We should also say that most men would want to have sex first (though they won't admit it!), if it wouldn't hurt the woman, because most men access love through sex.
If someone wants to have sex early, it doesn't always mean that they are ONLY just looking for sex. And, you know, some "serious" guys who go very slowly, step by step, may seem responsible, but they can also have other bad motivations-or maybe they are going slow with you because they already have a secret mistress.
So, the answer is that you need to practice judging people! Not by using your ideas of politeness, or by using social customs or standards from your culture. That doesn't work because people have different customs and cultures. Do it by testing his empathy, and by seeing his reactions to others. See if the man really cares about your feelings, if he likes you, sees you as a real person, has a sensitive personality, cares about your safety and and so on. This is not related to how soon he wants to have sex.
I recommend that you stay open to guys who want to have sex early. Try to see if maybe there is a caring part to his adventurousness, or if he really is just selfish and lustful. If you think he might be good and like him, but you aren't ready for sex, show that you are willing to try to become physical. For example by just starting to hold hands after a date or 2, and see how that feels for both of you. Seeing how holding hands feels for both of you can tell you a lot, and also motivate honesty. Some guys who only want sex and nothing else will be scared away by it. Of course, you must do this in a safe environment.