Fundamentally, I believe people are against gay marriage because of Religion and because they think it's Icky. Neither of which are valid reasons to deny rights. We don't bar 70 year olds from marrying people young enough to be their grandkids (Hugh Hefner just got married to a 30 year old). That's WAY more icky then two lesbians getting married.
People against gay marriage should just suck it up, be honest (you know actually follow what Christ said you hypocrites!) and come out with their real reason.