I just recently had intercourse with one of my male co-workers. He was just so fine

and easy to talk to and one thing lead to another. No one else who works with us knows and we behave the same around each other. The thing is he wants to continue "having relations" and I'm not so sure that I do. I know that I shouldn't have let it get to this point, but how do I tell him I want out without changing things?