Arguments against gay marriage

I would like to know any reasonable arguments for denying marriage to same sex individuals. The arguments I've seen don't hold water according to our constitution and bill of rights. Here are the arguments I've seen:

1. It's in the Bible.
   That doesn't hold up - our constitution and bill of rights ensure religious freedom and not imposing a specific religion through government on anyone. Our founding fathers were very clear on this.
2. It's unnatural.
   This also doesn't hold water. There is tons of evidence that all kinds of animals engage in homosexual activity and that it's part of the natural order.
   
3. It's a lifestyle choice not an innate characteristic.
   This would be the only argument that would justify the denying of any right to gay people. Obviously discrimination against characteristics that come at birth like skin color are against the constitution and bill or rights because they are inherently unfair. If being gay were a choice then you would not have to provide the same rights to a gay person because it's a behavioral decision. But clearly this is not true.
   

Despite what the vast majority of gay people say, that it is not a choice, straight people refuse to believe them. Well, if you're straight, ask yourself these set of questions?

1. Why would anyone in this society choose to be gay?
   
2. Can you choose to be gay? Would having sex with the same sex feel natural to you?
   If you are truly 100% straight the answers to these questions will be no. If you're 100% gay the answers will be exactly the same. Having sex with the opposite sex will seem unnatural and impossible. Many gay people live a majority of their lives pretending or wanting to be straight. If it were a choice why would this choice fail?
   
   So if it is not a choice, denying a right to a group that has an innate characteristic that is outside of their control is against the constitution and bill of rights. It seems pretty simple. People claim that the courts are undertaking a social agenda or social activism. I disagree - they are just enforcing the constitution and unfortunately for those who oppose gay marriage, the arguments are clearly not strong for their side.