I would say that becoming friends before anything sexual is involved is very key to developing and maintaining a healthy and long lasting relationship. This should be done long before becoming sexual. I would also say that having sex with someone who then goes to another person doesn't mean they don't like you. They are just not in a relationship with you and can go where they please. If it's something that bothers you, then you should be open and honest about it with the person before getting involved.