Marriage and morality are grey

I was just chatting to a friend from the US. He's been dating this girl for the past few months and recently found out that a good friend of his (who's married) had been sleeping with her, prior to her going out with my friend.

Chatting with him, I realised that these issues that used to be black and white are now slightly grey to me. My own code is that I won't sleep with or have an affair with a married man. But morality is not an absolute - it depends on the social mores of the era and the society in question. And who am I to judge other people when I don't know what their story is or if there are extenuating circumstances?

I see and hear about so many different marriages that are 'unconventional'. For example, both spouses have affairs and actually know the other parties, but stay together for the kids. There's even a family whose husband has fathered other kids outside the marriage and the wife is actually okay with it. I don't know if divorce would be better (would the kids be warped in such a household?) but if it works for them, that's great.

So then, what constitutes morality? Or I guess my question really is more, where does one draw the line? It's a slippery slope once you've crossed a boundary. Say previously you would not think of even going out with a guy who's with someone else. But then, you think about it and say, well, they're not married, and it's his choice, so why not? Then you end up sleeping with him. If it's okay to sleep with someone even though they have a girlfriend / boyfriend, then what's to stop you from sleeping with married people (oh, their marriage is not working anyway)?

With the erosion of one boundary, it's so easy to rationalise and erode another. It's like starting with stealing $10 from a friend, then you easily do it again, increase the amount, and eventually rob a bank. Where do you stop? And what constitutes extenuating circumstances? I stole the $10 because I needed it to feed my starving family is okay, but I stole $10 because it was fun, or because I needed a drug fix is not?

While we may have our own set of principles or morals, these morals can eventually be broken down, subverted and even perverted. There may be circumstances or other factors that need to be taken into account too. But I think the main point is that we have to be aware of this slippery slope and hold on to some principle or ethics or surely we will be lost, with no compass or baseline to assess ourselves.