Well, I have seen dogs that hump other male dogs. This might be fascinating if it weren't for the fact that I've also seen them humping a pair of shoes.
I bet there is a human somewhere who has also done that
Do your own examples of cats and inmates not demonstrate that in certain circumstances (i.e. lack of opposite sex partners) homosexual behaviour will in fact occur naturally?
Anyway the notion that natural always = good is silly.
Your arguments seem not only contradictory but also heavily dependant on the naturalistic fallacy.
What has the naturalness or otherwise of a particular behaviour necessarily got to got to do with whether or not it is immoral?
Is watching TV natural? Is it immoral?
Is unprovoked violent behaviour natural? Is it morally right?
There are many homosexual Christians and there are many homosexual non-Christians. It doesn't seem helpful to impose your own very narrow view of what is natural, what is right and what is Christian on this young man seeking support and advice.
Frankly natural, morally right and Christian are completely unrelated things that you are unthinkingly combining to = shouldn't.
If he is gay he is gay and there is no should or should not about it.
He just is and should be allowed to get on with being who he is.