In my mind, the problem is with religion. If it didn't say in the bible that marriage is between a man and woman, no one would care as much as they do now. Children go to Sunday school and learn this stuff and being children, and being sent there by their parents, they are brainwashed. This is a direct result of putting some book written 2000 years ago by a bunch of farmers to be some ultimate moral compass. In fact, it is full of oppression.
But in the bible it also says this, and even in the most religious households in the US, no one in their right mind believes and practices this anymore:
5:22-6:9pp -- Col 3:18-4:1
22 Wives, submit to your husbands as to the Lord.
23 For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior.
24 Now as the church submits to Christ, so also wives should submit to their husbands in everything.
25 Husbands, love your wives, just as Christ loved the church and gave himself up for her
26 to make her holy, cleansing her by the washing with water through the word,
27 and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless.
28 In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself.
29 After all, no one ever hated his own body, but he feeds and cares for it, just as Christ does the church--
30 for we are members of his body.
31 "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh."
32 This is a profound mystery--but I am talking about Christ and the church.
33 However, each one of you also must love his wife as he loves himself, and the wife must respect her husband.
Ephesians 5:22-33