Do people usually cheat down?

My boyfriend cheated on me with a woman in his friend group (apparently, he did not tell her about me). I looked her up on social media and she is attractive (she models), is quite educated, has a great career, has a great group of friends, and seems like a well-adjusted person. I’m just wondering why her now that I know that she isn’t ugly, dumb, etc.
Do people usually cheat down?
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