Marriage used to be similar. It used to speak to the circle of life.
Your marriage may still speak of the circle of life. If that is what gives your marriage meaning, that can be respected. In fact, I see that as quite beautiful. But that has only to do with your marriage and with those who share your wonderful view of marriage. It has nothing to do with anyone else's marriage; and neither do their marriages have anything to do with yours.
The continuation, increase and progress of the human family.
Gay couples are a family. Many are adopting and raising offspring. Some bring offspring from previous heterosexual encounters and include them in a family. There are many orphans and children worldwide who need better homes than what they currently have. They can find that home and find a family with two parents of the same gender. Or with a single parent. Or with heterosexual barren parents. Or with heterosexual "traditionally married" parents who wish to give a child opportunities they wouldn't ordinarily have had.
It raised hopes for the rise of a new generation.
That's not marriage. That is procreation.
That's why in almost every culture - even where marriages are arranged and there is no love involved - marriages are usually the biggest festivals and are celebrated greatly.
Each celebration took place for different reasons. As an example, an "arranged marriage" between Dark Ages/Middle Ages civilizations sealed agreements of ending wars, cease fires and alliances. So the belief that the celebrations were always about the married couple may not always be accurate.
People get married but don't want to have children.
Maybe they have good reason. Maybe one is emotionally challenged and they don't feel they would be good parents. Maybe their society is troubled to the point that they don't wish to bring children into the hardship. Maybe they believe that their financial resources are inadequate to give a child the future they deserve. Maybe one or the other carries a genetic defect that they don't want to risk passing on to a child. There are many reasons why a couple would choose to not have children. Maybe some of these reasons are actually selflessness, instead of the selfishness you perceive them to be.
Cheating is no longer considered a crime.
But even in places where it's not considered a "crime", it is still considered "immoral".
A husband has no duties to his wife and a wife has no duties to her husband.
In many societies, the spouses decide for themselves what each other's duties to each other are. While this may not fit with tradition, this may have a beauty of its own.
You can get divorced for any reason.
This is because of misogyny and concerns of domestic abuse. It may not be an entirely different thing.
Indeed gay marriage is quite a late arrival to the devaluation of marriage. But it does devalue it nevertheless.
I fail to see how my hypothetical (sadly nonexistent) same sex marriage devalues your marriage; which you have described to me to have a stunning depth and beauty of its own. My own marriage -- the commitment between two people who wish to be partners and see each other through the joys and pains of life to the very end; regardless of and blind to gender; has its own unique value that is separate and a non-statement on the value of your marriage.