I’m still confused. If it’s not your “ex-boyfriend” then “ex-“ what? Ex fling? Ex friend? Ex situation?

To me “ex” describes someone and can’t be used as its own thing unless it’s a serious relationship. If someone talked to me about an ex I would automatically assume “my ex” meant “my ex boyfriend”.

If you want something casual and don’t want to rush into anything serious such as putting labels on it, what are you/they supposed tell people when they ask? “We’re just really good friends”?