Who should be able to marry? What about Christians and non-Christians ("do not be un-equally yoked")? In any event, despite whatever your personal views, preventing gay marriage is a losing battle.
The biggest protest against gay marriage is what I've heard called 'the sanctity of marriage'. It's true, marriage done right is sacred. Sadly, marriage in America hasn't been sacred for a long time. Divorce rates are at an all time high, and even in heterosexual relationships, open and polyamorous marriages are becoming more common. A man and a woman being married does not offer their relationship any innate protection from immorality and sin. The sancity of marriage is based on a relationship as God has designed--if your only requirement for what makes a marriage blessed by God is the genetalia of the participants, then you are setting yourself up for disappointment.
The fact remains that regardless of what the majority of the people may want, as homosexuality becomes more acceptable* [It will. If you believe it is sin, then a sinful society will embrace it. If it is natural, then the barriers are nothing but prejudice], you will begin to see fewer arguments for marriage being a right for some, but not all.
The only viable solution that I see for saving the word "marriage" (since that is what we are arguing over. It is impossible to mandate people from boinking whichever consenting adult they choose.) is as follows: the goverment will no longer issue marriage licenses. The state will offer civil unions to any two adults who wish to enter into a long term partnership. Romance will not be a factor, however, each adult can only have one such partnership at a time. These unions will cover ALL rights previously afforded to marriage--legal next of kin, shared resources, tax breaks,etc. (Civil unions will also be as difficult to dissolve as divorces are currently.)
Marriage will become a solely religious ceremony, with no legal implications.
Remember, compromise. If you feel the need to regulate who people boink, and are up in arms about a word, then you need to give something to get something. Gay people can have all of the benefits (and drawbacks) of "marriage", but they won't be able to use the word "marriage" as your church defines it.
Meanwhile, focus on other avenues of protecting and strengthening relationships. When we raise mature and independent adults who have the ability to commit to and keep their promises despite adversity; when two people prepare for a life changing decision-aligned in their unchangeable beliefs and compromising on all else, bearing with each other in love; when we offer our support to couples and use our relationships as open examples, sharing difficulties and triumphs; when love is treated less like a feeling and more like a committment and a sacrifice; and when the benefits of a long term partner, dearer to you than yourself is celebrated--when all of this is the norm of a marriage under God's blessing and created by His template--then and only then, can we say that we have protected the sanctity of marriage and have earned the right to fight for the word as well.