I do agree and understand that "moral decay" is a broad term, with many different topics, factors and opinions involved. However morals are related to what is right or prudent based on a lesson or information from an experience. Regardless of the logical slippery slope the social construction view of marriage creates, reality reveals that marriage is a fixed, natural institution, incapable of being redefined. As a rule, as a group, and by nature, a man and woman in a long-term committed relationship bring forth the next generation. Society is built upon this unique institution. Therefore, when we look at the union of a man and woman and the critical role it plays in society, we call this marriage. We did not create marriage or define it, but rather give a name to something that already exists.
So the truth is, it is not culture that constructs marriages or the families that marriages begin. Rather, it is the other way around: marriage and family construct culture. As the building blocks of civilization, families are logically prior to society as the parts are prior to the whole. Bricks aren't the result of the building because the building is made up of bricks. You must have the first before you can get the second.
Our society has attempted to redefine the cultural meaning of marriage, but whatever additions they included will not be marriage, indeed they cannot be marriage, for marriage is a natural institution uncreated by man. In the same way, paper and plastic can be eaten, but that does not make them food. People may observe and celebrate this wonderful institution; however, they have no right to redefine it the way they did.