Whether one attends such a wedding is a matter of circumstance and conscience as far as I'm concerned.
As far as calling evil good and good evil, I think that is precisely the problem. The vitriol and hate which many Christians spit toward people is, as far as I'm concerned, a far grosser sin than that which they are condemning. When we are called to bear witness to the love of God which is in Christ for sinners, and instead we dehumanize our neighbor and punch down we are not bearing witness to the Gospel, nor are even preaching the Law--we are simply ourselves acting as transgressors of the Law.
When Jesus speaks to the woman caught in adultery, how does He treat her compared to how He speaks and treats the hypocrites who weaponized religion?
It isn't the prostitutes and "sinners" that Jesus' ire was targeted against, but the religious elite who spoke frequently of God's Law but were self-righteous hypocrites.
If you cannot in good conscience attend a same-sex wedding, then don't. But to suggest that a Christian sins by attending one is to add to God's commandments something He has not said; and when it comes from a place of condemnation and a misguided sense of righteousness then it is not an act of Christian piety but rather an act of religious hypocrisy. And it is in that right there that we find "calling evil good and good evil", because as depraved sinners we will find any way we can to justify our own rotten and carnal behavior. For as the Lord told us, our deeds are evil and we despise the light and prefer the darkness, and so we have already fallen under judgment and condemnation--which is why we can do naught else but trust in the mercy of God which is in Christ for us sinners.
-CryptoLutheran