first question you need to answer for your dilemma is would you prefer no contact or a friends with benefits arrangement. maintaining contact by a friend with benefits arrangement slightly increases possibility of rekindling your relationship whereas no contact will end your relationship with him. do you enjoy sex with him or just gave in to have sex because you thought it would reestablish your relationship? are you ready to walk away from him and search for a relationship with someone new? once you have firmly answered these questions you will know how you should respond to him in the future. he will surely try to continue the friend with benefits arrangement with you. without a decision by you for no contact there is little motivation for him to rekindle your mutual and exclusive relationship.
No need to get worked up about it. Say no again. You should keep saying no until he understands it. I get what you’re going through. The problem between your ex and you is miscommunication.
He is under the impression that the sex after breakup was because you want to be friends with benefits with him. It’s your responsibility to remind him that what you both went through is just break-up sex. Almost all broken up couples have break-up sex. It’s just the release of pent-up feelings.
Try not to think too much and don’t put a lot of emphasis on this encounter. Free yourself and say no with all your strength. Make him understand why you had sex and end things as quickly as you can.