by 
First, a word of encouragement: if you are the kind of friend that a gay couple would invite to their wedding, then you are probably doing something right. When Jesus ministered, those who were despised by society, the tax collectors and the sinners, drew near to Him (Matthew 9:10; Luke 15:1). He was a friend to them.
Further, no one sin is greater than another. All sin is offensive to God. Homosexuality is just one of many sins listed in 1 Corinthians 6:9-10 that will keep a person from the kingdom of God. We all sin and fall short of God’s glory (Romans 3:23). It is only through Jesus Christ that we may be saved from sin’s eternal consequences. (Please see What does it mean that Jesus saves?)
Some would contend that a Christian should have no qualms about attending a gay wedding and that one’s presence at a gay wedding does not necessarily indicate support for the homosexual lifestyle. Rather, they view it as extending Christ’s love toward a friend. The thought is that one’s presence at a wedding ceremony is an act of love and friendship toward the person—not toward the lifestyle or spiritual choices. We do not hesitate to support friends and loved ones who struggle with other sins. Showing support and unconditional love could open doors of opportunity in the future.
The problem is that a gay wedding is a celebration of two people who are living a lifestyle that God declares to be immoral and unnatural (Romans 1:26-27). “Marriage should be honored by all” (Hebrews 13:4), but a gay wedding dishonors marriage by perverting its meaning. Unlike weddings of those in other faiths, a gay wedding does not qualify as a marriage, according to what God declares marriage to be. A marriage between a non-Christian man and non-Christian woman is still a marriage in God’s eyes. It is still a fulfillment of the “one flesh” relationship that God intends (Genesis 2:24). Even a marriage between a believer and an unbeliever is a valid marriage (1 Corinthians 7:14), even though God commands believers to avoid such marriages (2 Corinthians 6:14).
A gay union is not a marriage in God’s eyes. God ordained marriage to be between a man and a woman for a lifetime; to take that holy and blessed union and link it to something God declares to be unholy is unconscionable. How can we ask God’s blessing on a union that He declares to be unnatural?
Suppose a Christian could attend a gay wedding and somehow communicate clearly that he is supporting only the individuals getting married and not their lifestyle. The individuals he is supporting are still holding an event which celebrates their immorality. There is no way around the fact that a gay wedding ceremony is a celebration of sin. We support an alcoholic friend by helping him refrain from drinking, not by going to a bar with him. We support a friend addicted to pornography by making him accountable and getting him help, not enabling him to hide his addiction. In the same way, we support a homosexual friend by helping her out of the lifestyle, not by signing a guest book at a celebration of homosexuality. We do not truly help our friends by attending an event where their sin is applauded.
It is admirable to show love to a friend. It is good to seek opportunities to witness to and show kindness and love to our gay friends. However, such motivations are misguided when it comes to attending a gay wedding. It is never our goal to drive our friends away from Christ, but Christians have a responsibility to stand up for righteousness, even if it results in pain, division, or hatred (Luke 12:51-53; John 15:18). If invited to a gay wedding, it is our conviction that a believer in Jesus Christ should respectfully decline and find other ways to express love, respect, and care.