I think I was quite clear as to why a marital relationship is a private matter.
I think we can agree that in society there are many different moral systems. Now I will use the words "good" and "bad" with the stipulation that these words have no inherent meaning, but instead only have the meaning that we apply to them from within our own personal moral system. You can compare another moral system to your own, and decide that it is "better" or "worse" but you cannot make an objective determination. Now, you can decide that your morality where adultery should be prohibited/prevented/punished is better for your purposes than mine where adultery is none of the government's business, but that is just your personal preference. The problem is that this may never end and there is always another fellow who is more moral than you or I or both of us, and whose morality tells him that shoes are immoral or that red-haired people are witches or something like that; and this fellow wants the government to segregate the witches for the protection of the good moral folks, and also prevent everyone from wearing shoes, even firemen. So, we have to compromise on our morality and find a set of common rules upon which most of us can agree and which will protect people from serious harm even if it allows for some behavior that makes some of us uncomfortable.
Why do you think that two other peoples' marriage is some concern to you, or to society at large? And please, no references to vague concepts that we have no real definition of such as 'making the world a darker place'
>For me, that would be very nice indeed! Convincing moral arguments are very rare!
That's because morals are very personal, arbitrary, and often irrational.