I suspected that analogy would orbit over your head, so let me explain.
It describes the infinite futility of ifs and buts. If his mother had a dick, she would have been his father. Clearly that 'if' renders the whole concept defunct, as his mother, physiologically would not be his mother if she was male with a dick.
Do you see how the concept works ? By supplanting an 'if' into the context you can come up with virtually anything, regardless of how far it reflects the reality of the situation.
It's a bit like saying 'If you had any knowledge of football, people might have some respect for you'