a couple things to say in GS's favor...
We've always struggled with this from an ethical perspective but yet there's pieces of all of us that acknowledge that these connections were real and important and that in reality (as long as it stayed under the radar) it was highly beneficial to everyone. The 3rd party was happier & more fulfilled and therefore their home life was actually better that it was prior, when there was stress from lack of fulfillment etc.
There is a principle, called
harm reduction often used in treating drug addiction and other high risk behaviours, which basically accepts the reality that people are going to do bad things in dangerous ways if we don't give them a safe way to do them. So, they are given a safe way to do them. Teenagers are given free condoms (I'm not against teens having sex, but I think that even people who are tend to approve of this practice.). Heroin addicts are given clean needles. It's not a perfect solution, but it's better than doing nothing.
So if we accept that the person the cheater is cheating with isn't actually responsible for causing the cheater to cheat, and that the odds are that he/she was going to cheat with someone no matter what GS did, and we know that the home life has gotten better since the affair started, then what GS has been doing is actually a form of harm reduction. If the cheater had gotten involved with someone who had no respect for their partner and their relationship, who wanted to try and take them away from that relationship, things could have turned out much worse.
For all we know, GS has been saving marriages.
I'm not saying it's ok to go after married people just to try to fix their marriages, but if you can reduce harm in the life of someone who is hurting, why not?
Another point is something that was mentioned by someone else already but I can't remember who... The fact that some relationships only function with a certain level of "don't ask, don't tell" in them. And some people prefer to leave it at that. My deeply Christian parents (who I'm living with presently while I'm back in school) recently found out that I'd had casual sex with a woman. Before it had a chance to become a full-blown confrontation about my lack of values, I offered my dad two options: openness without judgement, or "don't ask, don't tell." He never got back to me on the subject, which saddens me, but I understand. It would break my mom's heart to know everything I believe about relationships. And she's not a critical thinker so it would be very difficult for her to even make sense of it. My point as this relates to partners is that although we get to choose our partners, some choices are better than others. Not everyone chooses someone that complements them. Some partners are very dependent and fragile. If my dad ever cheated I would hope that he would keep it hidden, for my mom's sake. That may elicit gag reflexes from a lot of you, but you don't know my mom. She wouldn't make it on her own.
Lastly, having sex with someone who is cheating on their SO feels really good. Getting head from a girl while driving her boyfriend's brand new Audi through downtown in broad daylight is even better. The hedonist in me can't ignore that fact. I think people should avoid cheating, but if she's going to do it anyways, I might as well enjoy it.