There are many now, in light of the California decision to open the way for same-sex marriage, who would say that we are wasting time and resources to resist same-sex marriage in our nation. They would say that we are fighting the inevitable. I do not agree. Number one we serve God who can do as He pleases, and if it is His good pleasure He will turn our culture back from this brink of insanity. Second I believe that marriage is so important it is fighting for and seeing saved. But this response demands a why answer, why bother. I think there are a number of reasons.
First, God has defined what marriage is for us, we are not free to recreate marriage as we wish.
Second, for the good of homosexuals. Sexual sin (all sexual sin) seems to be especially enslaving and we long to see homosexuals flee the destructive power of sin and know the joy of knowing and following Jesus.
Third, for the good of society. All societies need the rock-solid foundation of healthy families to have a healthy culture.
Fourth, for the good of children. Children need both a good father and a good mother (not necessarily biological, I'm all for adoption), anything short of this will lead to some form of imbalance in the child.
Fifth, for the glory of God. God is honored when His creation follows His right ordering of human life. This is most true of His own children. He is honored when we stand for truth and the good of others.
Last, for the cause of the Gospel. When we refuse to call sin sin, we undermine the Gospel. If people do not see their own sinfulness then they will not see their need for a savior. Let us stand up for the Gospel and call sin sin, and then let us show love for the sinner without compromise on the truth.
If you have anything to add feel free to do so in the comments. If you more and/or better reasons than mine I would honestly love to hear them.