He wants to win you back
Truth is a man may actually want to have s*x with his ex because he wants her back. s*x releases a lot of powerful emotions which can be difficult to refuse. Hugging alone releases a lot of chemical reactions that elucidates bonding, talk more of naked s*xual romps. Thus, his plan with having s*x with an ex could be to get back in the game. If you aren’t interested, stay away from s*x with the ex.