Juni,
You seem to be going through some personal marriage issues here and carrying it over into your posts.
From someone who just lost a 33 year marriage and a 38 year committed relationship, this is a hell of a lot more complex than whether or not you'd share a toke or a drink with someone who had been unfaithful in a marriage.
Is remaining faithful in any marriage the ultimate test of morality?
Is it your or my responsibility to judge this in another couple's life?
You still see things in terms of sin and guilt, which indicates you're still fully locked into the Christian world view. If you widen that world view, will you begin to see things differently, apart from the sinner and victim mentality that haunts Christianity?
Lots of things change over several decades in a relationship. At what point does it become more ethical to move on than to remain in a relationship that has lost much of its meaning and is detrimental to both parties?
Eternal vows of love and fidelity are artificial constructs that we've been taught are an ultimate morality - yet human experience in every culture known to humankind proves over and over again that it is simply not so. We create an ideal romantic vision of true love and forever soul mates that is far removed from any reality I've ever seen. Yes, two people can be really committed and happy, but that is not always the case. Unless we've walked in all the shoes of all the other couples of this earth, how can we judge everyone on what is the right or wrong thing to do based on a religious cultural ideal that we've come to accept as the final statement on the subject?
Things are just not that simple. I'm no advocate of adultery, but I also understand that it is not quite as clear cut as you seem to think it is.
S4