OK I'll justify it. When we are nice to each other, like kids saying yes sir and no ma'am to adults, opening doors, saying thanks and just generally being respectful makes life more enjoyable. Enough people where I'm from agree with this assertion that manners have been normalized in society.
Hi I'm not going to reply to the matters more related to gay marriage as, of course, that is highly likely to result in thread derailment. The point was Nico's also, so it's up to him if he wants to follow up on it.
I think you are misrepresenting cause and effect here. Those today who agree with the assertion that politeness norms make life more enjoyable were also those who were taught those norms from birth. In other words, these norms don't govern because the population of the moment think they are justified. Rather, the population thinks they are justified because they are the norms that govern. When you ask someone; "why do address your father as sir?" they will give justifications but not the actual cause of their behavior--that they were taught to.
Now, I will have to point out a few things so my position won't be misunderstood here. Firstly, I'm not saying codes of courtesy cannot be justified--only that the justification used should be sufficient in itself to warrant the code, with no credence given to tradition in itself, and with a full awareness that, initially, and with no small persistence, one's intuition will move to defend what one has been taught thoughtlessly. Secondly, just because I am saying that justifications usually given for norms are insufficient, or that norms can be unjustifiable generally, I am not saying that norms were never justified at one point or another. As you say, they usually had a benefit to society (note, not necessarily a benefit to individuals, truth, or other societies) that justified their existence in the first place.
Over the years civilizations figure out what their people like and what maximizes social benefits to the populace. This is why I'm somewhat traditional, because I'm not willing to throw out centuries of trial and error and cultural evolution because some johhny come lately thinks he's the first person who ever had a good idea.
And of course my previous leads on to discussion of this point. I do indeed recognize the process of cultural/social evolution that has occurred and which will continue to occur. Two points.
1 Somewhat tritely I'd like to point out since extensive, particularly free self-examination and questioning of cultural holy cows is a relatively new phenomena, it hardly seems fair to call its proponents "johnny come lately's". They and their sort had much smaller scope for such activity historically, and a such their influence over society was less. They were late to the party mostly because nobody wants to turn up at their own funeral early.
2 Here is my more interesting point. We both agree that something similar to a cultural evolution has taken place over the multiple centuries in which societies, cultures, civilizations have had the chance to evolve and compete amongst one another but it seems after this we have a division in view. I think you've mistaken the object (which is to say the beneficiary) of societal evolution. It is not the happiness or wealth of individuals nor, directly or consistently, even the majority of the population that society would propagate. Society benefits itself. A "fit" society is one that continues to exist or expands. Napoleonic France, surrounded by royalist enemies, survived and expanded because its culture allowed and encouraged revolutionary new methods of conscription, not because it made its people happy. The British Empire controlled huge areas of the world yet did so through the suffering of the majority--the working classes under the horrific new working conditions of the industrial revolution. Oppression, fear, bigotry, ignorance, lies, delusions, persecution, slavery-- these are all so much more useful for a society to enlarge or continue its existence than for its people to be happy or have their wishes met. And evolution is about survival. Norms are the genes of society; they have developed because they help a society survive and spread itself--"goodness" is going to be secondary (which is not to say they wont be good).
Which makes it no surprise that many norms were horrible, and that other norms are unjustifiable now that notions such as freedom and truth have taken hold. Maybe--controversial opinion--norms from the ages of say, racism and slavery are worth interrogating. Again, I feel the need to point out that I am not actually claiming that our common codes of conduct don't have good justification under modern lighting, only that a thorough examination (the first stage of which might well be seemingly trite complaints going on in this thread!) is a worthy objective.