We could probably address your question better if you answer Bob's question:
- What specifically do you want to argue?
Nevertheless, my answer would be to base a discussion/(not argument) totally on biology and the subscience of biology: Physiology. Physiology is a subscience of biology that deals with how living organisms (like a man and woman) interact with each other, a (man or woman).
If I were talking with a gay man or woman who was an atheist, the last thing I would do it talk to them about Catholic morality.
Because no gay atheist is going to believe any Catholic doctrine or teaching you bring up with them. Atheists believe in dogma but just one dogma: There is no God.
What they can't deny is the physiology of how they were created; the scientific aspect of who they are as a man or woman.
Try a Google search on: physiological differences between man and woman
The physiological differences between a man and a woman explain why certain homosexual acts bring on illness. Without being crass, when gay men or women start putting parts of their bodies into other people's bodies, where they don't naturally belong, the physiological aspect of that person's body kicks in and says, Hey, this doesn't belong here! and illness follows.
So my advice is to argue based on something they really can't deny: science: like biology and/or physiology. I'm sure there are many pieces on the physiology of the human body that back the Catholic viewpoint.
In a previous answer my colleague Mary Ann said:
Second, I would make sure he knows the truth about the gay lifestyle and its effects:
- short lifespan
- health problems
- domestic violence
- the inclination toward the young [by survey]
We also have a recommended reading section. Mary Ann highly recommended some of these books.
That's my view on your question.
My colleagues may have other views,