That's called the bait and switch my friend. bait with sex, switch with relationship. Women seem to have to always be in a relationship with somebody to feel good about themselves. So they find a guy they like, let them in to their pants, then try to weasel their way in to keeping you for themselves. I would reply with something along the lines of
"I don't get in relationships with anybody I haven't known for XX months. I don't really know who you are and I'm not going to just dive blindly into a relationship like that. How do you think so many relationships end up failing miserably?"
If she counters with the "Well what about the sex? Didn't that mean anything to you?" or something similar, you need to counter with something like
"Well, no, actually. I don't put an emotional importance on sex. Sex is fun, something I enjoy, but I don't get emotionally involved just because I'm having sex. If I did that I just end up hurting myself over and over again, that's not very intelligent now is it?"
You can change up the words or however you like, but the fact is sex does not equal relationship. trust equals relationship. and if you don't know a person, there should be no reason why you would feel the need to trust them with your heart. remember to always protect your heart.