Ethics and morality are worldview dependent. So, its impossible to separate them from each other.
The re-definition of marriage is what people have been opposed to. That definition comes from God himself in the book of Genesis: this is the claim they believe in. That is why they see it as an attack on their faith. Even people who do not hold to this definition of marriage see it as I described.
Oh, you want to talk about the redefinition of marriage?
The Musuo culture of China has "walking marriages" where anybody who expresses interest in each other has sex in secret, and share little responsibilty. Men are expected to take care of the children of their sisters, not their partners. Lineage passes through women.
Vietnamese culture requires a procession from the bride's house to the groom's house, as well as permission from the bride's family.
In India, especially in the older days, most marriages were arranged. If the husband died, the wife was cremated along with him. Children were commonly arranged off to be married, especially for politics. The wife's family is expected to pay the husband's family a "dowry". The married must wear the traditional dress, even if the guests are wearing western suits.
The Ancient Greeks didn't believe in a formal ceremony for marriage, just that the couple should mutually agree to it. Women married in their teens, men in their 20s. Men often kept prepubescent male lovers.
In contrast, the Germanic peoples believed that women and men should be ideally marired in their 20s, at a similar age. This "age similarity" concept still exists in modern Anglophone culture.
Traditional Chinese culture required consulting extensive family trees to ensure there was no incest, no matter how distant, but only on the father's side. Marrying the non-immediate relatives on your mother's side was okay.
Medieval Christianity had no formal marriage system, but simply consisted of mutually agreeing, declaring your intent to marry, and then having sex.
Buddhists consider marriage completely secular, and define no religious rules for it.