Is cheater always a cheater?

I know a guy who has been dating a girl for a few years. They seem to love each other in every social media pictures. He got her pregnant and now they are engaged, with a one year old daughter. For some temporary work situation, they are not living in the same city. I found out he is on dating app. I wonder why does he do that? Does he not love his fiance enough? Or he just wants to enjoy the last bit of single life? Will he likely continue to cheat once they live together?
Is cheater always a cheater?
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