She didn't consider his feelings at all, and acted solely to serve her own short-term interests of getting laid. This makes her scum, worse really, but scum can suffice for the purpose of this explanation.

Aaaaand here's where you lost me again. I see this behavior as callous. Disrespectful. Hurtful. And human. Forgive me for being blunt, but you calling them scum? That's an effort to elevate yourself by denigration; to sit on your high horse and feel superior because that's all you've got in that situation. Someone cheats on you, your ego gets bruised, so you lash out with all the rationalization you can afford. That's what I see all around me. People going to DEFCON 6 because OMG THAT CHEATING BITCH WHORE FUCK HER SHE SHOULD DIE CHOKE ON SOME MORE DICKS!!! I'ma saw all our things in half, that'll learn her!

Now, should people take a step back, have a rational discussion (if possible), and probably end things? Abso-fucking-lutely! The trust is broken! But at least try to do it like goddamn adults, okay? And if that doesn't work, do what you gotta to save yourself. But don't let the hate fester. And don't make yourself the villain by being an obstinate prick unless it's truly warranted! Remember that intentions do matter. And cheating is usually just a matter of fucked priorities--at some point, the desire overpowers the duty. It's not usually personal, they just care less for your feelings than they do about them getting theirs. Yes, it's selfish. But we're inherently selfish creatures. Expect it, understand it, and you won't feel as quick to hate people for it. Hate the sin, not the sinner, right?

Now, if things are being done out of spite? Well, then it's time to get the hell out of Dodge. I had to chew off my leg with another GF, before the cheating one, but she turned out to be certifiably damaged. Like, committed-by-her-parents damaged, and manipulative to boot. I felt sorry for her for a long time, but that kept me there, and I reached a point where talking wasn't accomplishing anything. Those are the times when it's best to break contact, because you're legitimately protecting yourself. But not all relationships that end with infidelity are like that, and I think it's silly to automatically assume they are and then behave as such.

I dunno. I guess, when it happened to me, I felt the need to earnestly ask, 'Why?' Not in a, 'I'm trying to guilt you by sobbing when I say it' way but as a real explanation for things. Like, talk to me, we've been in committed relationship for several years! What made you do this thing? Granted, I knew all along. Or maybe it just seems really obvious now. In any case, I have a clean conscience because I didn't act the fool prematurely.

Now here's the double-standard laid bare, though, for those of you still reading. Do you cut everyone out of your life that betrays your trust at some point? Your best friend? Your parents? Or do you talk to them and try to find some closure? Maybe try and keep things civil if not friendly? Yeah, exactly.

TL;DR: There's the Reddit Reaction™ to infidelity. And then there's the real-life, morality-is-shades-of-grey, human reaction that I hope most people opt for. Don't automatically eschew the latter for the former unless there's a good reason to. Otherwise? You're probably being a dick and trying to rationalize it because hurt feelings.