"Such were some of you; but you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus Christ and in the Spirit of our God." 1 Corinthians 6:11
"Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest." Ephesians 2:3
Gay "marriage" is wrong because God says so! Homosexuality perverts the picture of the Gospel that marriage was made to portray. Having said that, Jesus loves homosexuals so much that He died to save gay people from their sin, too, the same way He died to save murderers, rapists, child molesters, liars, thieves, adulterers, etc., from their sins.
Every single Christian who has repented of their sinful practices and believed on the Lord Jesus, trusting Him for their eternal salvation, used to be a wretched, vile sinner. We used to practice the same things the world practices and condones today. Pay close attention to the words of these two passages of Scripture:
"Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God. Such were some of you; but you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus Christ and in the Spirit of our God." 1 Corinthians 6:9-11"And you were dead in your trespasses and sins, in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest." Ephesians 2:1-3
Notice the past tense of these passages? Every single person on the face of this planet is dead in trespasses and sins. Because of our unrighteousness, we ought not to be deceived as none of us would inherit the kingdom of God. As Jesus said in the Gospel of John, "Truly, truly, I say to you, unless one is born again he cannot see" or "enter the kingdom of God" (John 3:3, 5). Genuine born-again Christians used to walk in the lusts of their flesh, indulging the desires of the flesh and mind, until they were washed clean, sanctified and justified, through the Word. They used to walk in the same sins as those mentioned in 1 Corinthians 6:9-10, as well as others not named there. We were the same as every other person on this planet.
Homosexuals know that what they practice is disgusting and perverted, regardless of how much they try to convince themselves otherwise. Their conscience continually reminds them of it. So there is really no need for us Christians to constantly remind them of their sin. They know! What we Christians need to be doing is preaching Christ Jesus to them. If you examine every sermon preached in the book of Acts, do you know what they all have in common? Jesus! Jesus was preached without ceasing. Every sermon was about Jesus. The early church proclaimed Christ and Him crucified (1 Cor. 2:2). It is the Gospel that is the power of God for salvation (Rom. 1:16). Again, while this seems so simple, the fact is, God's methods are always simple. We just need to trust them and obey Him. If you want to see souls saved, God glorified, and the culture changed, start preaching Christ and Him crucified.