One philosophical viewpoint would be that this is circular reasoning. Both the concept of hetero marriage itself and the idea that "gay is a sin" are religious ones.

It would be a sin to disagree with you on this point. I agree.

You could argue that from a non-religious evolutionary/procreation standpoint that homosexuality is abnormal. But I don't think any gay couple expects to be able to conceive offspring; their lifestyle is what it is for non-reproductive reasons. So that argument would be circular as well.

This is where it gets a bit dicey for me -- not uncomfortably dicey, mind you, but just a little dicey. From the whole procreative standpoint (at least in the human species) heterosexuality would seem to be where it's at. 'Course this assumes that all 100% of the citizens have to be procreatively humping away for the species to succeed....

Then, while you'd think that [no] gay couple expects to conceive offspring, I'm not sure that's the case. Where did that neat little "Gayby on Board" tag come from on that baby carrier? And that whole turkey baster thing tells me that some women are quite determined to engage their maternal instincts without the messy complication of....Men!!