"Marriage is between a man and woman according to the Bible."...or...
Why? (Also, entirely irrelevant to the law.)
"Because homosexuality is a sin."
"Because it's an unnatural abomination."
"Because it's gross."
"Marriage has always been between a man and woman. It's just history. And tradition."...or...
"So we should respect tradition that works and not redefine a historical institution."
Even when that means it discriminates against 5-10% of the population?
"Marriage was designed for child rearing. Gays can't have children."...or...
They can't? Adoption? Surrogacy? (Also, the elderly, infertile, or asexual can't marry?)
"Oh, we're against those too."
"Because studies show two married birth parents are ideal."
So we're only allowing the ideal situation through the law now? No divorce? No parental death allowed? No out of wedlock pregnancy?
"Ideally no, but we can't legislate those things."
"Because people wouldn't support it."
So you're only enforcing the ideal when it's against a small minority you can beat politically?
Oh, ok. How's that working out for you lately, by the way?
"Marriage shouldn't be redefined. But civil unions are ok."Instead, the conversation should go more like this...
That's big of you. So they'll have all the same rights, but not be able to use the term "marriage." One might even say you're allowing a "separate but equal" version for a persecuted minority group.
"Exactly. See how accommodating and open minded I am?"
Yes. I'm sure the people who assigned special water fountains for black people felt the same way.
"Hey! This is different!"
Do you accept that gay people do and will always exist?
"Yes." (Otherwise you're totally out of touch with reality.)
Do you accept that they do and will always be able to adopt or have their own children?
"Yes." (Otherwise you're ignorant and spiteful.)
Then wouldn't it be better for society if they raised those children in a loving, stable, married home?
"Yes." (Otherwise you're contradicting yourself.)
So you support same-sex marriage then?
"Yes." (Otherwise no one should ever consider you trustworthy on anything ever again.)
Thank you. You may now be considered a reasonable human being.