Essay:The problem with homosexuality
The problem with homosexuality is that it has nothing to do with marriage. It violates God's intention, which is for everyone to grow up and get married. In general, religious people try to avoid anything having to do with love or sex unless it is supportive of this intention. C.S. Lewis remarked that masturbation has nothing to do with marriage. Pornography is generally held to be not conducive to marriage. Extra-marital affairs and premarital sex are considered sinful because they lead away from marriage.
Obviously, lying about your relationship (by saying you are married) cannot turn "living in sin" into marriage. There are exceptions in law, whereby a man and a woman who live together as if they were married for a period of time may be considered to have entered into a common law marriage. This is not to absolve them of the need to seek benefit of clergy or to justify their fornication, but to protect the rights of the woman and the rights of any children she might bear.
The problem cannot be solved by declaring that people of the same sex can get married together. This is simply an instance of denial: mislabeling the situation. Only the most immature of people try to get away with this sort of self-justification, like a thief saying he wasn't stealing ("just borrowing your car, guvnor; I was going to bring it right back").