So from my learning/reading, it's bad to talk about past relationships on first date because lets say their relationships ended poorly (cheating, etc.) then you don't want them feeling those negative emotions with you. You also don't want to bad mouth your exes.

But then I think about it after and isn't it smart to try and find out how/why their relationships ended if your end goal is a relationship? You don't want to waste your time and money going on several dates before finding out they cheated on their partners (of course they wouldn't tell you this, but you get the point).