So it's solely a case of morality that prevents gay couples from getting married? Reading your position, I thought ok then, but I started having trouble thinking of solely moral laws. All the big ones are moral, but also involve taking away another's rights (life, liberty, property, etc.) and so are justified on that basis. Drug laws might be solely moral, but there are strong cases for legalizing some of the less harmful drugs (alcohol was made legal again for instance) plus not having your population die is pretty much an essential function of government.
All the laws I can think of right now have at least some semi-valid justification outside of strictly saying "it's wrong".
Looking back through history doesn't help your case nearly as much as you'd like either. Homosexuality (at least between males) was common in many ancient cultures, including Greece.
Looking at nature, there is evidence that other species include homosexuality.
Using your argument, perhaps we should consider why so many groups hated Jews over the years, or perhaps why so many groups treated people of dark skin as slaves (really it was originally any foreigner, but those of dark skin were definitely foreign in many parts of the globe and so were slaves), or why women were considered inferior in many cultures.
One thing for sure, you're right that homosexuality is not a new thing. That alone says something for how 'natural' it is.