What should i do about this man, that i am infatuated with.?

You like a guy ALOT and will do anything for him, you become really good, close friends and he flirts with you and then you and him end up sleeping together. So OBVIOUSLY you think your going to go out and then he turns you down for a girl 4 years younger then you. BUT he keeps asking to sleep with you and telling you how beautiful you are eventhough he has a girlfriend...what should i do?