They cheated because they could, and they knew they could.
A less attractive guy won't mess around on you because he's just not built that way. Men with more testosterone are consistently rated more handsome than other men, and men with more testosterone are 38% more likely to cheat.
It's perfectly fine if you want to date/sleep with someone just because they're ridiculously good-looking.
Sometimes dating an unattractive person will make you realize that having a partner who physically turns your crank is vitally important to you.
But, every now and then, a couple pulls the switch off this genetic equilibrium.