Look,
The argument goes like this:
P1. Were either theism or naturalism true, then any action taken by an agent is either naturally necessary, random or teleological.
P2. Homosexual actions are not random.
P3. At least some homosexual actions are very probably the result of natural necessity.
P4. Were God to exist he would posess at least the following traits:
A. Loving humans.
B. Having created humans as naturally heterosexual.
C. Having a desire for humans to freely choose a relationship with him.
then.
C5. From 2-4 it is very unlikely that God exists.
P6. If naturalism were true, then humans and animals are very probably more similar than under theism.
C7. Therefore we would expect to find natural instances of homosexuality in other animals.
P8. A posteriori: We do find such instances.
C9. Therefore it is both very unlikely that theism is false and also we observe an instance of evidence for naturalism.
Bingo bango.