Expound on this theory a bit here. I want to hear it.
Well, you see, the whole concept of an "immaculate birth" always struck a poor chord with me. Even in old myths where gods had kids with mortals, they always had sex. Never just pointed there finger and said "Boom, you're preggers, have fun." So I got to thinking, what if Mary wasn't actually a virgin? What if, in order to try to hide her shame, she pretended it was some miracle, and gave birth to some stranger's baby? I do recall that most women were required to be virgins in order to be wed, and if I'm not mistaken, in the bible it says that if you rape a woman, you can pay her father and marry her. I may be wrong on that, but I digress.
Anyways, moving on from there, we get the birth, which is now expounded into a big deal because "it is a child of God, not of Man." Imagine being a kid raised to believe you're of divine lineage in a simple time where things like DNA Testing doesn't exist and people are a lot more spiritual than they are now. And every day you're bombarded with this, so you start thinking to yourself, "Hey, I'm pretty special. I should do something with this." So then he goes about preaching about how he's the only way to reach his Father, and how he may be god incarnate, which made him a traitor to his lands governments, and resulted in him dying the typical death of the time (crucifixions weren't exactly uncommon), and turning him into the martyr that served to form an accepted cult/religion. Then as time went on, more and more people gossiped, mythos from other places got tacked onto this guy who nobody properly recalls because everything is secondhand gossip, then many years later, we get bibles made and even later on translated.
And this became the Judeo-Christian lore, also believed by Catholics and to some extent Muslims.