Is it possible to remain just friends with an ex? Would you allow your SO to do it?

su

I was reading an argument on fb. Pretty much all of the comments said that there’s no reason for you to remain friends with an ex unless you plan on going back to them at some point. And how men and women can’t be “just friends”

I just don’t see the big deal. it’s possible to have a healthy friend ship with your ex while in a new relationship. One of my Bestfriends is my ex. Granted we were friends before our relationship trial. my fiancé loves him and has actually never given me any issues

Just wanting other views.