"not conforming to conventions: behaving in ways that contradict conventional assumptions about what constitutes normal or acceptable human behavior"
Is this really a good argument though against gay couples adopting? Your leaps in logic seem to leave a pretty wide gap, IMO.
"Eating mashed potatoes with ketchup is unnatural, so two men shouldn't be able to have kids."
All the food we eat, even the fake, canned, GMO stuff, is
of this planet. It's possible because it's there and we have the ability to do it. You seem to be talking not about what's natural and unnatural, but what's conventional and unconventional. That's a slippery slope to go down if ya ask me. Some other really "natural" and conventional things of their respective times: Blacks counting as 3/5 of a person, blacks not being able to vote, women not being able to vote, blacks and whites using separate bathrooms, segregated schools, etc.
Sometimes mores and folkways are broken because they need to be. I don't think clinging to what's "natural" is the best argument against gay couples adopting.