What is A genuine reason for being against gay marriage that does not include hating the concept of gays, or the concept of marriage?
Two adults want to share their life, with the burdens and rights that non gays get.
Really, why would you care if not because you find the concept of same sex couples bad and harmful?
Marriage yields children. Making new people grows a civilization. The state would definitely want to incentive any means to grow a civilization. In the modern day, these incentives look like tax deductions. Gay marriage doesn't yield children. Why should the state give the same incentives?
My solution: The State shouldn't create incentives for people to get married. People will get married because they want to.
I can give a silly example in order to avoid restarting the debate. Someone who truly believed that gays would cause earthquakes could vote against prop 8. purely on the basis of the San Andreas fault. If you think for long enough you could probably come up with less and less silly sounding arguments, until eventually you arrive at one that a rational and non-evil person could convincingly be misled by. The essential point is that even if you are 100% convinced that the Mozilla CEO held an incorrect belief, it still remains to be shown that evil was the only explanation for that belief.
You could never prove evil.
Because it's a matter of values.
You could easily say, that holding negative opinions on something strongly enough that you act upon them, without any proof of that thing being dangerous or bad is a sign you are prejudice about that thing.
Now being prejudice is not evil in my opinion, but if you value being prejudice more than finding out the actual reality of your negative assumptions, makes you pretty much the text book definition of a bigot.