This is hard to argue against if you are not on board with the whole deity concept.

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.

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.

Food for philosophical thought. Doesn't necessarily have any bearing on a given community's laws regarding same-sex marriages, though.