It seems to me like mom had an affair, unless Dad was lying. He told Julia that mom was in the hospital due to complications with the birth. On the other hand, maybe it was a mental breakdown at the same time as the birth, and Julia IS adopted. Leon lies so much, it's hard to guess which parts of the story he was leaving out, and why.
If they are attempting to make him more sympathetic wrt the deceptive sperm "donations" because he "needed to prove his virility" due to his wife cheating, that's not going to work with me.