I think you BOTH used each other and got the recreational sex you BOTH wanted. Now you want the guilt you're feeling over it to be his fault, and I suppose that's normal. But it's not fine. He didn't rape you, he didn't manipulate you with false offers or anything. He showed you some attention and you climbed into bed with him.
You had sex again because you think that's the way you interact with a guy for fun. You were both horny, you are both familiar to one another, so you both participated in that outing. You BOTH used each other, OK? Own that.
Calling him an "ex" ultimately is the same as calling him "a guy I tried to have a relationship with and things don't work between us." If he were a stranger, sleeping with him might even make MORE sense since the stranger might make a good match for you and hasn't actually been tested on that point yet. Your ex and you tested each other on it and failed. So what's the deal? Since I don't recommend sleeping with strangers (who might actually BE a good match for you), I definitely don't recommend you sleep recreationally with a guy you KNOW already you don't work well with.
So, the only question is since this guy now holds the same equality in life to you as the postman, the BurgerKing boy and any other guy who you see and talk to but aren't "in a relationship" with...are you going to keep sleeping with non-boyfriends?