Have you read, or heard of, the book “Making Gay Okay”? What you said sounds like it was pulled right from the pages.
The principle is that God created marriage to be both unitive and procreative. That is the basis of natural law, and why God made man, and then when He made woman from a man’s rib, He didn’t make the woman identical to man. (If God wanted to make woman identical to man, He surely could have, because God can do anything and it was HIS creative work, not anybody else’s! God could have easily made everybody identical, and we’d be able to reproduce by ourselves, just like the cells in our bodies. Remember, God was not obligated to follow any rules when He created everything. But He did what He did.)
Back to my point. Marriage was designed by God to be unitive and porcreative.
When divorce was legalized, you can kiss the unitive (until death) aspect of marriage goodbye.
Once we made birth control and abortion legal, the procreative part of marriage flew right out the window.
So now, what do we have left? Marriage became just “a thing people do.” God’s design for it had been thrown in the trash heap because of what I just said.
Add in a government which treats married people and single people differently (over 1,000 laws on the books legalize differentiated treatment–everything from tax codes, to estate rules, to housing laws, and all kinds of other laws which probably shouldn’t be on the books as they are, but they are), and BAM! Now marriage becomes a civil rights matter, and civil rights cannot be denied to anybody on the basis of race, religion, gender, or SEXUAL ORIENTATION! Therefore, the court system had no alternative other than to legalize same sex marriage.