“Many believe that the human population is the greatest threat to the Earth.”
I can see their point.
“2. Human reproduction occurs naturally through a man and a woman.”
Through? Man and woman REPRODUCE, they’re not vectors for reproduction.
“3. If zoophilia is not in conflict with secular humanist morals, then it is a valid alternative for these atheists.”
Zoophilia involves fucking creatures that are unable to provide consent. It’s in conflict. BIGLY.
“4. If more secular atheists would practice zoophilia, then human populations would reduce.”
Secular does not mean amoral.
“5. Therefore, practicing zoophilia is a valid and pragmatic opportunity for secular humanists to help reduce the Earth's population.”
But it will not reduce the population. The density of atheists is not that big, the density of zoophile atheists is not big enough to put a dent in human reproduction values.
“Which point or points do you disagree with?”
Starting with two, your phrasing sucks.