As I attempted to suggest earlier there are different definitions of the term "natural" in play here. To the Catholic the natural order is that morality applies only to humans so other species behaviours are irrelevent and amoral rather than immoral.
So it's a parallel definition of 'natural' just like their parallel definition of morality (sin), because why use language normally when you can keep your own 'special' dictionary and claim that you're therefore infallible whilst making deliberately vague statements in an attempt to justify dreadful institutional homophobia (and misogyny and, historically, racism).
To the naturalist, morality is basically irrelevent since they are just talking about behaviours.
I don't know about 'naturalist', that's more a field of study. To Deontologists the behaviour is not that significant in a moral sense, it's the intention behind it.
The naturalists difficulty is the association of the word natural with common and majority behaviour.
No, the problem here is that Catholicism is trying to redefine 'natural' to exclude the entirety of the rest of nature and claim, without justification, some sort of human exceptionalism that makes us different in quality rather then merely in scale.
Then there is the appeal to nature where goodness or otherwise
Which no-one was making. No-one was saying that it's moral because other creatures do it, they were saying the Catholic stance is in error because can't qualify it as 'unnatural' when it's so prevalent in nature.
O.