We all commit sins, no doubt. But I think a real Christian finds that his sin is a weakness and a vexation to him. He takes no glory in it. He is saddened and shamed by it. He stumbles---but that's what it is---a stumble. It is not his preferred method of walking through life. Society does make the standards and they do change with time, but the presence and influence of God within that society is determined by its overall righteousness. If you deliberately accept error and wrongness into your way of life and fellowship, when you pray to God you will be looking up to a brass ceiling.
But when it comes to sexuality, the particular problem is that it is a persistent, daily recurring want. Jesus was clear: remarriage after divorce is adultery. And yet all of the Orthodox Churches, and virtually all of the Protestant Churches, allow remarriage after divorce - EVEN THOUGH IT IS ADULTERY according to Jesus (THAT part CANNOT be honestly denied - Jesus said it) - as an allowance to human weakness. It's a fact: remarriage after divorce is adultery - Jesus said so.
It's also a fact: people will not simply be celibate for the rest of their lives just because they got divorced.
It's a fact, too, that the Church has always known: if you make the rule such that people cannot remarry, they will fornicate, and they will turn away from the Church. The Catholics have held the line on no remarriage after divorce, and the Catholic Church has declined and diminished on account of it.
Eventually, the Catholics vastly expanded annulment, and vociferously claim that it's NOT Catholic divorce, convincing precisely nobody other than those Catholics who want to believe that.
So, ultimately, ALL of the Christian Churches, as a matter of mercy and as a matter of bending the knee to human reality, permit remarriage after divorce (which they sometimes call "annulment" and loudly protest it's NOT divorce, even though it really is in most cases) EVEN THOUGH it means that the remarried people are adulterers per Jesus.
Reality is the Church will do this, or it will die out as people abandon it. All of the Churches know this. The Anglican Church came into EXISTENCE over the matter of divorce and remarriage. The Orthodox Churches will permit up to two divorces and three marriages before they give up.
The Catholic Church will keep finding grounds for annulment - and when it doesn't, Catholics remarry anyway and leave the Church.
One can kvetch all one likes, but human sexuality is too strong and too insistent and too persistent, and men are too weak, to not commit sexual immorality.
Fact is masturbation is the same sit as fornication, which is the same sin as adultery, which is the same sin as homosexual sodomy. It's all "inappropriate contenteia" on Jesus' short list. And it's ALL forgiveable by God - and God had better forgive, because not one normal adult is free of this sin.
Given those realities, it is relatively easy to see how the Church CAN, if it decides to, extend the same merciful allowance for human weakness that it does with divorce or annulment and remarriage. The spirit may desire to be perfect, but men and women are not, are not going to be, and if a Church tries to hold them to sexual purity, the church itself dwindles, and the faithful fall into sexual immorality ANYWAY, so nothing is accomplished.
Christians already know this and make allowance for it in the case of masturbation and remarriage after divorce/annulment, and the regularization of relationships that begin in fornication through blessing those relationships in marriage. To apply the same grace to homosexuals is not a thing of a different order - it's all equally the sin of inappropriate contenteia we are talking about: sexual immorality. There is nothing WORSE, sin-wise, about homosexual inappropriate contenteia than autosexual masturbation. The minds of the conservative like to imagine there is, but those vain imaginings are not true, at least as far as what Jesus himself said.
So, the Church CAN get there, and to protect it's tax free status, and for Christians to protect themselves against anti-discrimination laws, the churches will eventually get there. When they do, it will be along these grounds too.
That's the legal argument. That's how it can be reasoned out. Gay marriage is no different from remarriage after divorce: it's tolerating sexual immorality. The Churches have tolerated sexual immorality for a couple of thousand years because of the realities of human weakness, the need for mercy, and the desire to survive. During those years, social norms allowed them to take a harder line against certain sins (e.g.: Homosexual immorality and witchcraft, and interracial relationships) than others (e.g.: masturbation, fornication and remarriage after divorce (adultery)), and they did. The social norms have changed and the Churches have changed with them, in order to survive.
Gay marriage is the next change coming down the pike. In 100 years all Churches in the West will have a version of it. Between now and then there will be terrible storm and stress over it.
In a similar vein, in 100 years the Catholic Church will have married clergy, because it has to make that change in order to survive.
Some things are so, whether you like them or not (and if you're reading this on this thread on this site, it's a very good bet you DON'T like any of this at all - nevertheless it is so).
Don't kill the messenger.
Scratch that: you CAN'T kill the messenger. Don't attack the messenger. I've told you the truth. I've shown you the future. Read it. Absorb it. Realize that, before the end of your life, you will be making the mental adjustments necessary to move in the direction of tolerance. Dislike it as much as you must, but thank me for showing you the way that you will eventually tread in order to stay viable in society, and to keep up with your churches as their clergy, to preserve their jobs and their institutions, make the necessary adjustments to survive.
That is all.