Because, you can't say being gay is natural so you accept it on those grounds, and then turn right around and ignore the millions of other things that happen in nature that you condemn (murder, cannibalism, genocide to name a few).
That's grade-A Christian bible "cherry picking" you're quick to point out and condemn, but now the shoe is on the other foot, and you're willing to cherry pick yourself.
Hypocrite!
Secondly, I think every tax-payer should be able to do what every other tax payer does. That's not an argument FOR gay marriage as much as it is a reason why I don't fight it.