This is very interesting question, to me. Yes, I am an rationalist, but I was Christian before I decided to use my brain. And I still don't understand where the supporters of Prop 8 get their hate from. So, religious nutjobs of America, WHY DO YOU CARE?! Regular religious folks can answer too. Here's a list of common answers, and my rebuttals
Gay sex is icky- They don't want to have sex with you.
The bible says its wrong- The Bible says a lot of things, and the bible was written half by Rabbis, and half by priests in the first century. Also, why would you care if they sin? If one man sins, it dosent mean his neighbor will go to hell too.
Marriage is a Christian Institution- Marriage is a HUMAN institution. Other religions have marriages, even atheists get married.
THINK OF THE CHILDREN!- This is one of the many issues where the children are brought in where they aren't effected at all. They are just dragged in by demagogues. They don't talk about straight marrige in school, why would they talk about gay ones?
An I promise, that, like Rush Limbaugh, I will keep half my brain behind my back, but unlike Rush Limbaugh, I am not high on prescription meds.