It makes no sense that you would break up with someone you clearly didn’t get along with and then go be intimate with that same person. If you want to keep hooking up, then why did you break up in the first place?
If you know you have your ex on the side, you won’t feel like you should go out and find someone who you really connect with or really like. You will get used to merely tolerating someone for the sake of a hookup instead of actually appreciating someone and expressing your feelings via intimacy.
If you want to be friends, you are making it that much harder.
Clearly, if you are able to still hook up with your ex, there is some sense of civility in your relationship. Being civil is good for a friendship, but hooking up is not. Keeping the physical aspect of your relationship on top of being friends is basically dating. You aren’t learning how to keep things platonic and are ruining your chances of ever being friends again. Then when you stop hooking up, you not only lost your ex as a partner, but also as a friend.
It is easy to misinterpret hooking up as potential for getting back together. You can talk and agree that you just want to keep things casual, but that’s not going to stop you from thinking there is still a chance for something more. That yearning for something more is a very empty feeling, and the fact of the matter is you will probably never see that official relationship status with your ex again.
If you eventually move on from your ex and try to build a new relationship with someone else, things are going to get weird when you get that occasional “Hey! Wanna meet up?” text out of the blue. Temptations can be powerful. If you are trying to be happy with someone else, having that lingering ex is not the best idea.
Maybe you’re the one left behind. Your ex found someone new and now you have no one. Being single is not the worst thing in the world and meeting new people isn’t difficult, but suddenly losing something so constant it hurts. You’re not dating, though, so what did you expect would happen?