You are right, we need to pray and witness to the lost and dying world. They need to hear the "Gospel of Jesus Christ." And we need to show them the love and mercy that Jesus came to proclaim. But we are also told to teach the law and to establish the law. Romans 1:18-31 clearly shows the results of mankind turning away from God. We are called to be light and salt, and to redeem the times. Standing up for what is right is not something we should do as Christians, but it something we are required to do as believers. Praying for someone, showing them the mercy and love of Christ, is not approving of their sin. When Jesus healed someone, or made them whole, He said more than once, "Go and sin no more." Jesus also defined a marriage as a union between a man and woman. This is where we must stand, because we serve the one who gave us marriage. Marriage is also a picture of our relationship as the bride of Christ with Jesus.
Let His people stand, fast, pray, and then act to save the very basic institute that society is built upon, the biblical mode of the family.