A lot of people do. In out culture, sex is often shown as the ultimate reward. People who don't know how to look and dress are out of luck. Having sex is something people take for granted once a relationship reaches a certain point.
People expect it.
Telling someone you like that you don't want to have sex would be hard or seem weird. Sex is fun, exciting and normal. It's part of being independent and making your own decisions. You wouldn't have to look to far to find people who agree with these points. Refusing to have sex could drive away someone you really care about.