Marriage over the centuries has had many changes but always comes down to one constant. A man and a woman. Everything else tries and fails.
Untrue. Marriage is an institution that has existed at least since the dawn of civilization and probably longer, but that's what we have records reflecting, so say about 8,000 to 10,000 years. Over that time, the norm -- meaning the arrangement that has prevailed in most places and times -- has been polygamy: one man allowed to have more than one wife. Not all men did have more than one wife, because most men couldn't afford to, but all legally
could at most times and places throughout recorded history. The restriction of a man to a single wife is a relatively recent innovation, a few thousand years old at most, and prevailing only in Europe and cultures descended from Europe.
If you put all of our current cultural ideas about marriage together in a package -- wound up with the mythos of romantic love, an equal arrangement of partners for mutual support -- it's less than a hundred years old.
The issue of normalizing homosexuality isn't in a vacuum. There are many cracks and divisions in the culture that do not portend well for our continued existence as a stable society. This is just one. After all we cannot band together for the common good if we can't agree on what the common good is.
Our view of the common good is changing. We underwent a revolution in culture and values over about twenty years from 1964 to 1984, but that was only an acceleration of changes that have been happening for several hundred years.
Culture and values, like institutions, economy, and government, follow the lead of material circumstances.
From the time the first humans evolved on Earth (something between 100,000 and 200,000 years ago) until the dawn of civilization, human religious values and mores, as well as our ancestors' economic and political structures, took a constant, universal form. Religious belief taught human oneness with nature and subordination to nature, and our kinship with the animals. Gender relations were far more equal than they became later, in early civilization. Humans lived in small hunter-gatherer bands without formal government and with no private capital property; it was a lot like Marx envisioned as true communism.
From the time humans began to settle in agricultural communities (about 10,000 years ago), all this changed. The agricultural communities adopted private property, formal government, organized religion, an altered moral code that made man the master of nature (but subordinate to the gods) instead of being a part of nature, greatly lessened status of women as the need for high population growth took over. Eventually, all civilized peoples adopted these measures and almost all adopted a monarchical form of government with a hereditary landed nobility and a class of slaves or serfs at the bottom doing forced labor. During the transitional centuries, I'm confident that there were conservatives lamenting the departure from the old ways and predicting that society would run afoul of wrathful nature or some such, much as you are doing.
We are in a transition comparable to the one from hunting-gathering pre-civilized life to agrarian civilization. The invention of the printing press, then the scientific revolution, then the industrial revolution, more recently the computer revolution -- all of these things have radically altered our material circumstances and so required changes to our culture, values, and institutions. That's why the predominant form of government now is the democratic republic instead of monarchy, and many other changes have happened, too. One of the most important changes is that we no longer need to breed rapidly, and so we no longer require women to be subordinate to men. Gender equality leads to changes in sexual morality, as well as many other things, and also the lack of a need for high birthrates allows sexuality to be more separate from procreation and the non-procreative functions of sex to become more important culturally.
I can understand how all of these changes make a traditionalist like yourself uncomfortable. But they cannot be stopped. We are in transition from a classical paradigm of civilization to an advanced paradigm, and the old ways can never be brought back. They were dependent on the material circumstances for which they were developed and for which they were appropriate, and those circumstances are gone forever.