I am so disillusioned by politics, I don't generally get involved anymore -- and I have no intention of voting this year or in 2008. The two mainline parties have strayed so far from God's Word that voting has become a formality for many Christians. Then there are many evangelicals deceived into thinking that President Bush is somehow the epitome of godliness and Christian virtue. (I won't even get into that right now.)
The question posed in the OP implicitly assumes that abstention from voting on the issue of allowing homosexuals to marry is far worse than voting one way or another on the choices presented. Why?
As Christians, our job is to proclaim Christ and Him crucified to a world that loves death, and seeks to escape the only source of Life available to them -- the grace and mercy of the eternal God in His only-begotten Son, Jesus Christ Who came to die in the stead of sinners, and who sent His Holy Spirit to minister to the hearts of those whom He will enlighten and lead to repentance apart from any effort of their own or any intrinsic goodness in themselves.
This Gospel teaches us that the nature of our commissioning naturally excludes the attempt to fight these moral wars within the framework of secular political coercion. This is not a Christian nation at the political or legal or constitutional level, as much as Christians may wish it were. In those situations, politics be damned. We are charged with the task of bringing the love of Christ and the message of repentance and restoration to human hearts, just as the early Christians did in Rome.
Equally, neither are we to go out and actively SUPPORT this lifestyle by our vote. We are Christians who (last I checked) recognized the Word as God's divinely inspired and infallible standard and rule for faith and life and salvation. Our lives are to reflect its every teaching.
Our only option is the Gospel. And it is a Gospel that the Church must once again be willing to die for, if need be. Until we understand that, we are bound to remain an irrelevant, impotent, fundamentalist, introverted, legalistic, cult-like sect with an endless litany of "do's" and "dont's".
The Gospel has rules -- do's and dont's -- but it is greater than the sum of its parts, and is primarily characterized by the fact that it is the only religion which is capable of transforming and transfiguring the whole creation and everyone in it -- because the Word is more than lifeless words on a page; it is a Person, a living, breathing eternal Being who came down from Heaven to meet man on his own level by taking upon Himself human flesh, and dying as a perfect sacrifice to remove the sin and guilt of all those who turn to Him for everlasting life in genuine repentance.
We put our faith in a Divine Person -- the God-Man -- who, in adding human flesh to His divine nature, became a bridge of reconciliation between humanity and God. This is the one and only hope of our calling. Preach the Word, and let politics take care of of itself.