Because at the end of the day it's not really the sex that's the problem, is it?
It's the lying, deceiving, hiding, making-a-mug-of, covering up, paranoia-creating, etc, etc, ad nauseum that's the real problem when it comes to infidelity.
It's all this stuff ^^ that causes irreparable damage to a heretofore good relationship, and erodes trust to an unworkable level.
I have no wish to get into a slanging match with an OP who clearly has form (sorry, I don't recognise your name, but other well-known poster appear to). You come across as quite goady on this thread, but I'm happy to give you the benefit of the doubt.
If have extramarital sex is not that big a deal, then what's wrong with agreeing on that, instead of agreeing on monogamy?
This way, the true causes of the hurt and devastation - the lying and deception - are done away with. Why be astounded that, as a result of cheating and lying, your partner wants to leave you? Why shouldn't they? A relationship is not the be all and end all.
I say all this with no vested interest. I have never knowingly been cheated on, but am empathetic enough to understand how soul-destroying it must be for the cheatee.