"We can't, if we do 'that' you could get pregnant.  Abstinence is the only way to make sure 100%."

*Insert Pregnancy*

Joseph: Fuuuuuuuu.

Then he has to like grow up and shit- for those of us who were once babies, we know that growing up sucks, especially if you're the son of god.  Which is also hard as hell to prove.  "Let me perform this miracle right now, maybe then you might believe."  If someone tried to do that today, it would be called 'being a doctor of modern medicine' and people would just move on.

Then there's this whole feeling of knowing you're going to die.  He totally knows he is going to die one day in order to help all these people who have pretty much done nothing for him.  I've played enough video games to know the feeling of being near death.  Hey, that's what the health meter is for.  A crucifix, though, isn't in any any games that I've played, and I don't know about you, but being able to kill the son of god in one use seems kind of over powered.

Jesus RPG: Get to level thirty, die with no save points.

I'm sure there are other ways of absolving the sins of everyone forever.  Brainwashing every human being into drooling monkeys that are temptation-less, and who follow every command without complaint?  Nah, that could never work...