So, critically speaking, at the end of your life, I highly doubt that any higher deity is going to say, "Hey, you were a great person and you followed all of my rules, but you lived in a society where gay marriage was legal, and thus you are condemned to hell." Point being, I--and most other people with whom you are debating--are NOT judging you on your religious affiliation, or beliefs. I RESPECT that you have devoted your life to something greater than yourself, and I think that's a great thing.
However, I will NOT stand by while you don't show the same respects to other people. If you don't support gay marriage, don't marry someone of the same sex. I am fairly certain that if I chose to marry a woman, your purity/morality/etc. would not be compromised.
Furthermore, tattoos, females being subservient to males, cutting your hair, etc. are also forbidden in the Bible, but I see no one starting a campaign that is up-in-arms about these subjects. Gay marriage is INEVITABLE. If it doesn't happen now, it will within our lifetimes.
Although, sooner would most certainly be better for if you make all of the gay people wait to marry, they will just have their weddings after they have stable careers, and have acquired more money. Furthermore, their weddings will--I can assure you--be the most spectacular, flamboyant, and wonderful celebrations of gay love that you could ever imagine.
? Support Marriage Equality. ?