Question: Why would the Catholic Church say homosexuality is unnatural and against natural law when we see the same instincts in animals too.
When the Church teaches about natural law, it is about creatures with the ability to reason, not animals who cannot.
“The light of Thy countenance, O Lord, is signed upon us,”: thus implying that the light of natural reason, whereby we discern what is good and what is evil, which is the function of the natural law, is nothing else than an imprint on us of the Divine light. It is therefore evident that the natural law is nothing else than the rational creature’s participation of the eternal law.St Thomas Aquinas
If we want to consider animals as a basis for justifying natural behavior, remember they also eat their young. There is no need to go that far.
What we need to exercise is compassion:
I personally love ending such conversations on this note. Believing something is unnatural does not mean we have to hate the person. I say this to anyone who would want to justify any hostility or discrimination towards gay people to repent from their evil ways. We preach only love and peace.