Well that could be a whole nother thread all together, what exactly do you think legality should be based on then?
As I have been saying throughout this thread, rights, and the protections thereof.
I would say just about anything that does not hurt another citizen (besides him or herself) and/or take away from the rights of another citizen should be lawful.
For example, many states have motorcycle helmet laws. I am against that. I think if a person wants to ride a motorcycle without a helmet, they obviously don't have any brains to protect anyway, so let them. Good for the gene pool, too, because they're more likely to die and not reproduce. Seriously, the only person that person is going to hurt is him or herself. Some states don't require it, but if you are in an accident and you're not wearing a helmet, there is a limit to what insurance will pay. That's even better.
I used to be against seat belt laws for the same reason, but I did see a study that showed wearing your seat belt actually helps prevent accidents, not just protecting the person in case of an accident. If you have to swerve suddenly to avoid something, and you're not wearing a seat belt, you could be thrown by the force away from the driver's seat, and lose control more easily of the vehicle. Whereas if you are wearing the seat belt, you're more likely to stay in place, and in control of the vehicle. So, since it actually prevents accidents (which could hurt other people, not just the driver), I am in favor of seat belt laws. But not helmet laws. Again, I always wear a helmet ... because I have brains to protect. But if a moron wants to risk it, let them.
Similarly, yes, I understand that a gay person is "risking their eternal soul" by getting married. I've got news for everyone: a gay person is going to be sinning, whether or not gay marriage is legal. Allowing a gay person the right to visit his significant other in the hospital, or to live in the home they bought together after one has passed away, or any other of the more than 1,000 secular laws that are automatically given to married couples but not to "domestic unions," is not going to harm you or me.
Every study I've seen shows that in fact legalizing gay marriage will help the economy (through increased tax revenues ... I'll find a link if anyone's curious). So by fighting to keep it illegal, you're actually working against the poor and needy, against what Jesus commanded.
Seriously, folks, this isn't rocket surgery