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.

Their lifestyle in their context is for non-reproductive reasons. In evolutional context, it can also serve a cause. There are known species of crabs, where evolution has dropped the female part. Reproduction is based on cloning in the male body. Of course you've also got the hermafroditic types, like earthworms...