Do you have to be gay to support the right of gays and lesbians to marry? Absolutely not! No more than you have to be black to know that slavery is utterly wrong or Jewish to be sickened by the holocaust. We know that everyone deserves the chance to be happy, to share the joy of marriage with the person they love.
More importantly, we understand that marriage is about love, much than it is about sex. One needn't be married to have sex, but many couples feel the need to formalize their commitment to each other through marriage. It doesn't matter who you love, it's that you love at all that counts.
Lastly, we know that allowing gays and lesbians to marry has no effect on our own marriages and certainly no negative effects on society as a whole. How can celebrating love possibly be bad? We'll also note that it has been five years since Massachusetts began allowing same-sex marriages and they are still locust-free.