I think you mistake my position. I've never had a problem with gay marriage. I personally don't care. But I am giving you your answer to why this was a moral crises for some in America. The US adopted an institution that was uniquely religious. Even more so, they adopted the Judeo-Christian model of it. This was inherited from Europe. Marriage became important to society as a systematic process to distribute property. Remember Europe didn't begin to separate religion from governance until the discovery of people in the New World, and that transformation wasn't complete until the 20th century. Marriage became an important system for the transition and inheritance of property and culturally developed in most civilizations alongside agriculture when the concept of land and land ownership became extremely important. The Judeo-Christian vision of marriage likely evolved out of this as much of the old testament regards the importance of unadulterated blood lines (where the term adultery actually comes from) and monogamy.
When advocacy for gay marriage became a thing, Christian's misread that as an assault on their religion. To a Christian, "marriage" means a very specific thing. Mainstream society tried to change that definition. That's a really big deal. Think about it for a minute, in Catholicism, marriage is a sacrament. Can you imagine the reactions if main stream society tried to alter the criteria behind baptism? What was actually happening though, is society was adopting a secular view of marriage, which in all honesty, is sort of strange. Basically, mainstream society culturally appropriated it because for some reason people are in love with the romance (as in the perceived feeling) associated with monogamy. People find it "noble" or "honorable" and they don't really have a good understanding why. Science tells us "love" is actually a euphoria generated by hormones. Take that for what its worth.
Look at it from this perspective. Imagine if you will, the Arapaho Sun Dance, a distinctly native American religious practice. To what extent would we approve of main stream society suddenly adopting this practice, bastardizing it, and then changing specific rules, beliefs, and understandings about the ceremony. Then, to rub the salt in the wound, we tell native cultures not only is this still a valid form of Arapaho Sun Dance, you are required by law to recognize it as such. They are required to make Sun Dance cakes, and administer Sun Dance ceremonies for non-Native people all over who bastardize the tradition and the meaning.
That was the conflict with gay marriage in a nut shell.