From my experience, I don't think Thai women have to say yes when boyfriends ask for sex. You might think you care that person and never want to lose him. I would like to suggest you to be honest with your own feeling. Say "No!" if you don't want it, it isn't the right thing to do or not the right time. I am sure the nice boyfriend always respect you (if he is the "real" nice one).
Don't be afraid that you are not gonna have boyfriend, you'd rather have the good one that really love you than the one that looking just for sex. You might lose him because of saying "no", but you are not losing the respect of yourself.
I've been in the US almost 8 years and been dated 4 different guys (one at each time). I found this problem too. I had to break up 2 of them after 2 months dating because I refused to have sexual relationship. It was sad but I still have self confident. I went to relationship w/American boyfriend again. This time, I got the right one. He respect me and Thai Culture. We'd been in relationship for 3 years before getting married.
Well, everything depends on you. However, before you make decision to do anything, please listen to the answer from your heart carefully and care about your feeling.