This is very judgemental. Not all relationship is the same. I was friends with my SO's ex before I got to know my ex, got to know them independently of each other. I didn't know they had recently been an item because they weren't open with it. We talked about it like adults, all of us had the standpoint that you can't force someone to love you and that you should embrace it if it happens, our personalities are obviously compatible since we are all friends. My SO took a few years with very low contact with the ex but today we're all friends and hang out with the ex's new partner as well. I can say with 100% confidence that my SO is not romantically interested in the ex. We were all friends so why wouldn't we be now?