Melvin has a habit of making jokes that treat normal human interactions like business transactions. Earlier in the movie, when Nora asks him for a favor, he says "go sell crazy somewhere else. We're all socked up here."
Here, Carol knows that she is breaking normal social conventions by refusing so blatantly, and she might even be endangering her son's treatment by making Melvin so angry that he stops paying for it. Melvin, strangely enough, shows a lot of social awareness by defusing the situation with a joke. If a business is doing well, it won't open early--if a customer comes to them at 8:45, the people at the business will say "sorry, we don't open for customers until 9." By using that phrase, Melvin seems like he encounters so many people refusing to have sex with him that he has to establish business hours, and she needs to wait in line with all the other people refusing to have sex with him. It sends the message--"well, of course you don't have to have sex with me. That wasn't my goal."