Being friends with an ex is saying "We had this kind of relationship, that's over, but now we can START to have this other kind of relationship". And if you can do that? Awesome! I've done it and I've tried and failed to do it. It's not easy, you know? I am friendly with MOST of my exes. My husband is friendly with most of HIS exes. But we both have exes with whom we're not friendly.
His ex comes and stays at our house for 3-5 days a year, and that's great. I am friends with her too, it's a good thing. The fact that, a billion years ago, they slept together isn't on my mind. Meanwhile, one of his exes actually tried to break us up very shortly after we moved in together. Not cool! It is more about the person than it is about anything else. I'd kick her to the curb before I offered to let her stay for dinner. You get me?
I spent most of one of my good friend's wedding hanging out with one of my exes because my husband had a migraine and he was the only other person I knew and it was great! We had the best time. But it was old friends and career advice and how happy we were for our friends who got married...nothing more serious than that.
You can be friends with your exes. I've done it. It's good. But you have to be willing to settle for the shallow hull of friendship. You don't get a say in their choices, you don't get to butt in, you are a friend...again. Or for the first time. You. Are. A. Friend. If you can do that with an ex and everyone is chill and down with it, it is all for the good. You didn't date them/fuck them because they were bad people. But what you will have after the relationship, the intimacy, is over is...kind of hollow. It is the same kind of hollow you have with every friendship...I don't know EVERYTHING about you, I know what you'll tell me. It's not the same thing as being someone's lover, so you're held at arm's length or better.
NOT being friends is easier. That's always the advice. Don't be friends with your exes. That's great advice. It's incredibly difficult and emotionally straining to do it. It is absolutely the path of most resistance. You'll feel things you shouldn't, you'll want things you can't have, you'll miss the weird corners of it, it's not usually a good idea. But as someone who did it and did it well? It CAN BE DONE. It is just varsity level dating. It is the hardest thing to do. It can be done, but it takes so much skill.
You can do it if you want it. But...do you really want it? Do you want to be friends with him or do you want to be the good guy? That's the most important question. The desire to be the good person, the person who leaves it on good terms and makes no enemies and leaves no hearts broken...that's a fucking myth. You get that, right? You have and will break hearts. You will devastate someone's emotional landscape again and again. That's...leaving a relationship 101. You don't fix that by "being friends". That's bullshit. You already did the heart-stomping by that point. Being friends if just as way of saying, "When you're done feeling like a horse kicked in your chest, give me call and we'll catch up". It is actually harder than not saying anything at all.