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Rawksteddy
I see that you are a bit newer here. Don't let postings like the above give you the wrong impression about most forum members. Unfortunately, there's always going to be a few members that feel helpless in their own lives... and so they reach for the most overused canned-phrases and offer no actual constructive criticism (for a person with a low-self image, this offers them some quasi opportunity to feel 'empowered.')
Back to the subject:
Speaking about an ex on a current date isn't a good idea. This sub-communicates to the new date that you are still affected for the former gal. Also women are typically vanity creatures and crave recognition (this includes the ex that still had/has you as a social media friend .. and the current girl you've mentioned).
As far as women that cheat--- sometimes women are habitual cheaters and if their previous 'boyfriends' accepted this behavior, and as a result, they are more prone to do it again. Sounds like you put your foot down with the ex and demonstrated that you weren't willing to look the other way.