Several people have fallen back on the argument that if procreative self-lubricating sex is not distinguished from non-procreative sub-optimally self-lubricating sex by the word “natural”, then the word “natural” has no meaning. This is simply a very, very silly argument.
Of course the word “natural” retains its meaning. It retains many meanings. In fact, to the contrary, it seems quite impossible to me to construct some functional definition of “natural” that is based entirely on human behavior. I invite any of those advancing the “but, but, as a result natural loses any meaning” to tell me what the human-behavior-based definition of “natural” is.
The very point of this exercise is to show how the application of the word “natural” to the characterization of homosexuality is nonsensical. In fact, the only relevance that the word “natural” can have that distinguishes homosexuality from heterosexuality is in regards to this definition: “based on an inherent sense of right and wrong <natural justice>”. Here, you might found a point of defense, but I would argue this is one of the weakest and least useful definitions of the word.
There are otherwise any of a number of definitions, in fact, that suggest that homosexuality is by definition perfectly natural. To wit:
a : occurring in conformity with the ordinary course of nature : not marvelous or supernatural <natural causes>
b : marked by easy simplicity and freedom from artificiality, affectation, or constraint
c : having a form or appearance found in nature
d : existing in or produced by nature : not artificial <natural turf> <natural curiosities>
e : of, relating to, or operating in the physical as opposed to the spiritual world <natural laws describe phenomena of the physical universe>
f : having a physical or real existence as contrasted with one that is spiritual, intellectual, or fictitious <a corporation is a legal but not a natural person>
g : having or constituting a classification based on features existing in nature
h : having a normal or usual character <events followed their natural course>