Whether or not Christians should attend homosexual weddings is a sensitive topic. From what I see in Scripture, a Christian should not attend such a wedding since it would risk being a support and approval of something that is against God (Lev. 18:22; Rom. 1:26-27; 1 Cor. 6:9-10). As a Christian, and as an apologist, I must stand firm and represent God properly. Sometimes, taking a stand means to reject ungodliness even if it means being ridiculed.
Homosexuality is a sin and Christians are not supposed to support sinful behavior. But homosexuality is not just any sin. It is perhaps the only sin that when committed, God gives the person over to it. Consider this:
"For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, 27 and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error," (Rom. 1:26-27).
God's judgment is upon those who practice homosexuality. He calls it degrading passions and he gives them over to it. This is a judgment upon them for their rebellion. Should Christians then attend a ceremony that is sanctioning such a sinful practice? Of course not.
Homosexuality is also a direct rebellion against God and his design for men and women. In the garden of Eden, God told Adam and Eve to multiply and fill the earth. They were to demonstrate dominion in carrying out the command of God. This dominion is not to be oppressive nor abusive. Along with this dominion is the preaching of the gospel with its repentance from sin. But since homosexuality is a sin and since homosexual marriage cannot carry out the command of God to fill the earth, it is not something any Christian should support.
But it might cause offense
Unfortunately, when we stand for what is right, others might be offended. I heard a preacher once say that the gospel that offends no one is not the gospel of the Bible. It is not that we want to be offensive as Christians. It is not that we intend to hurt anyone. But we are obligated to follow Christ and not the ways of the world. Jesus said in Matthew 16:24 to deny yourself, pick up your cross, and follow him. He did not say to follow the compromising, sinful ways of the world. He told us to follow him. When we do that we will automatically risk doing things that unbelievers will not appreciate. In fact, they might ridicule you for your stand in righteousness. Well, so be it.