You're talking about Mary needing to be a hermaphrodite to begin with - a person born with an x and y chromosome set and who has a normally functioning womb (extremely rare upon extremely rare) - then for a situation where an egg is self-fertile and has two y-chromosome pairs - which is supposedly an extremely rare event for a woman's eggs to be self-fertile (although I imagine it is barely noticed when it happens as it would just be assumed it was a normal pregnancy in most cases).
Alternatively, we could assume Mary and Joseph did have a normal son and this explains all the effort in the early Christian texts to tie Joseph to the line of Abraham and Abraham to Adam. The idea being that Jesus was the true king and authority over Judea as a descendant of King David and why Harod flipped his shit at the prospect of newborns.
Jesus was not low-born. Though his upbringing was in the trades. While Joseph worked as a carpenter on the Temple, Jesus sat and talked/studied with the Pharisee.
The idea he was just some random dude who waltzed up and started hurling insults at the religious order of the time is kind of ill informed. He was known as a proper descendant of David and thus was part of the temptation to start an armed insurrection against the Pharisee. It was part of what the people expected of a Messiah - the people expected a king to rise up and lead them to military victory over Rome and even a conquest over their neighbors.
The reason why there is to be a second coming is because the same religious order would come to rule the world and, well, here we are.