Sure....I have a family member who always seems to complain and actually point short comings out in others. Also I engaged in a conversation that I should not judge gay marriages for I have sinned I'm sure with lust full thoughts which apparently equates to the same.
In this case pray for the person committing the sin. But you have every right to (for your own principles) to be very against gay marriage. It is wrong. We just need to keep asking God to intervene and make Himself very real to them and to get laborers accross their path with His Word, someone they will listen to. Forgive them just like Jesus would. That way if God ever desires to use you to speak to someone you are ready to accept the person as one of God's lost sheep and say what He desires for you to say to that person. Another good thing to pray is that God will open their eyes to the truth of His Word and grant them the gift of repentance. Because the devil has them blinded and in a prison and they don't even know it.
They are not committing a sin bigger than any other sin, because God sees all sin the same.....but if we don't pray and interceed for them and all unbelievers then they will stay trapped in their own prison.