Jan,
Ditto to double standards being a part of every culture, and religion seems to lead the way in that regard. I have Muslim and Hindu friends; the double standard which exists in those cultures makes the Western world look like we are all feminists.
The divorce statistics are relevant, as they have consistently reflected societal changes; a more 'telling' statistic is that there are more people choosing to be single. Cofty is right; ethics and how we treat one another are what is important. I've avoided this OP because they always turn into someone's pet peeve about sex. Sex is wonderful, natural, amazing, one of the best things that money can buy (just kidding) and when accomplished by consenting adults ---- is none of anyone else's business. Attempting to place pejorative adjectives on normal behavior is much more of a problem. We all understand how and why that occurred, but those conditions aren't relevant in most societies today.
Cheating on a mate, betraying a mate - these are ethical issues. Sex between consenting adults where betrayal isn't a part of the linkage isn't.
If that is what any of you call amoral, then I suggest you find a time machine and go back to the bad old days. Or keep your private parts - and your opinions about such - to yourselves.