Children are told in church from their tweens and teens: “God has the right (opposite-sex spouse) waiting for you.” Not biblical. In fact, it’s directly against the teachings of Paul.
We teach young girls that sex before marriage will damage them so deeply, that their future perfect spouse will not want them. There were a few men who didn’t mind the Samaritan woman’s “damaged goods.”
The culture and society treats adults without a spouse and children as if they’re only leading half a life without a nuclear family. People are taught that a greater worth or value is found for them in a good marriage. A failed marriage is a failure in character.
There is no greater worth or value in marriage than there is in being single. This is specifically supported by Paul’s writings, and something Paul had intimate knowledge about. We give him room to be short-sighted on this, but not on temple worship in a religion he knew (we hope) only rumors about? (If you're confused about this, I talk about that here.)
So the question should be: Why are straight people feeling threatened by gay marriage?
Has the cultural, societal institution of marriage been idolized and put on par with the gospel? Obviously, it has. Opponents of marriage equality have said as much.
We have idolized hetero-normative sex so much that we have built elaborate rituals around it. From courtship to purity dances to bridal showers and bachelor parties… it is its own religion.
Marriage has always been, and will always be a societal, social construct. Paul and Jesus and the Old Testament treated it as such. Yes, the two become one flesh, and it is a picture of God and humankind becoming one, but it is not the picture. It is not a perfect picture. It is quite an imperfect picture. It is the kindergarten play of the Broadway musical. A partner was created for dirt-man because he was lonely. Not because God wanted to get some satisfaction from viewing two people of the opposite sex frisking and fighting. Marriage is for us, not for God, and we need to stop pretending it's more important to God than it is to us.
Straight (or at least somewhat straight) people will not stop getting married and having cute babies because some people decide not to. I promise. Marriage equality is not a threat to your marriage, much less the gospel. And if a cultural, societal construct is a threat to your gospel, then your gospel is not a threat to the powers of darkness.
If we really want the Kingdom to come, we need to get used to looking with Kingdom eyes. There is no Christian or Muslim, no gay or straight, no male or female. We are all one. We all love, and sometimes we love another with such a passion that we want the privilege of committing our entire lives to another person. Two women can become one flesh. It happens, whether anyone likes it or not. That’s how love is.