Most Helpful Girl
I think that when you're cheated on, its pretty natural to compare yourself to the person your partner cheated on you with. Regardless of what they look like, you're going to feel badly, just in different ways.
If they are more attractive than you, you'll probably feel inferior. Perhaps like you weren't attractive enough to satisfy them and keep them from straying.
If they are less attractive than you, I suppose it's more confusing. What do they have that you don't? Why would your partner cheat with someone like *them*? It doesn't seem like ruining your relationship with someone like that would be worth it. Or maybe you question how attractive you thought you were. My guess it would be a sort of double-blow: Not only did your partner cheat on you, but they "traded down" to do so.
If the person is more attractive than you, its easier to rationalize that that's the reason they cheated, whereas if they are less attractive than you, its harder to understand (though, in reality, the reasons why people cheat can be far more complex than what the other person looked like, or could be as simple as "they were there and willing").