I believe in objective morality.
Porn is immoral.
A persons immoral behavior outside of work is a red flag. It means they are also MORE likely to ack immoral at work. Based on that, it could be grounds for dismissal.
Porn doesn't affect employee performance.
Not directly, and not more than any other outside activity.
Being of the opinion that someone should be fired or not hired because of something that doesn't affect work performance is a handicap on your leadership and collaboration skills. IMO.
You are basing your whole argument on an unsubstantiated assertion.
This is not an academic paper, this is an internet forum.