I'm not going into the US politics-related debate, but I think the first part of your post deserves some answers.
Nomorefun: If laws shouldn't be based on religious belief, what should they be based on? Personal experience? Majority rules? A couple of really old judges sit on a panel and debate what's actually legal/ethical, using their code of morals and ethics (grounded in their religious and personal beliefs)? I don't see how using basing an opinion on religious belief is any less valid than doing so on logic. At some point an individual has to make a logical choice whether or not they want to believe in a religion and try to follow it. Whether or not they represent themselves or their religion well is another matter entirely, but calling it invalid because their personal opinion is based upon religion instead of something else or what someone deems logical hardly seems fair.
So, in my view, you are basically saying that an argument being based on religion does not necessarily makes it illogical. In a way, you are right. We just have to look at why laws came to be in the first place, in primitive societies.
Though very probably informal, I can say they first appeared to improve cohesion within groups in order to improve survivability. It may be impossible to tell exactly when laws and religion mixed, but there are many guesses - to give an extra fear factor for those who might break then, to give more power to the religious leaders, etc. Anyway, we can tell that religious laws did, at first come from reason, all things considered.
And that was moral and ethics throughout most of history: whatever helped societies survive and prosper (or a few individuals who needed an ordered society to explore, but let's not get into that). It's not hard to think why homosexuality might have been outlawed in more primitive societies, as population growth was needed.
However, there is one thing reason and logic do account for that religions and traditions may not: the world changes, and so do the needs of organized societies. Christian law, for example, is based on a 2000 year old book. This book was supposedly written under direct instructions (or inspiration) from God, and therefore it's true and must be followed as it is, unless God himself comes again and changes it.
Now, religion and state, and therefore applicable laws, have been separated in the most developed countries. Laws should be, once again, based on reason and logic. However, reason itself has changed, mostly thanks to science. As survivability of any groups or human race as a whole is no longer an immediate issue, the focus is now on the well-being of the individual. Religious laws are way behind this new way of thinking. The ban on homosexuality might have been a residual tradition already by the time the bible itself was written.
And now we live in a globalized world, where you may find any kind of person anywhere, and information travels all around the world almost instantly. Compared to most of what came before this, the first world, and even a good portion of the third world, lives a golden age, brought by reason. And like it or not, reason, changing how the world developed and always adapting itself, now opposes religions, which have stalled. Not to mention religions oppose each-other, and that can only cause conflict in today's globalized world.
Finally, point is: laws, as dictated by religions, might have logic and reason at their background, however, it's an outdated form of reason, made to fit millenia-old societies and their own conflicts of survivability and power. Therefore, I call religious statements invalid.
How is passing a law based on personal belief any different than passing one based on religious belief? And if you can't pass a law based on either, how the heck would anything get accomplished?
As I said before, reason and logic, which is where even religious belief comes from in the first place. Of course, it's impossible to be completely impersonal, but that's what those terms aim for. And then, this is why they should be followed, whether you personally believe it or not.
As far as harm goes there is no substantial evidence one way or another to support that gay marriage will cause irreversible damage. Evidence will only present itself after two or three generations existing and living with gay marriage and by then it will be too late or everyone will scratch their chins and think, "Huh, what was all that fuss about 30 years ago? Things are awesome!". One side thinks it will do no harm and the other sees that only bad will come of it. Whether you base it on personal or religious belief it doesn't matter, reality is there is no concrete proof for either side, you just have to pick which side of the fence you stand on.
The problem is that harm itself is relative. Being able to see people of the same sex kissing in the streets is something religious people consider harmful by itself, as they fear their kids might do it. I, however, perceive a world in which sexual freedom is one of it's flags as a goal to be accomplished.
