It is rather simple, a particular religion believes in a particular story of how a certain messiah got magically popped into a virgin woman and therefor believe that the "soul" another magical thing is beamed in the egg right at the moment a sperm seed manages to push through.
Well could also be that our testicles are carrying millions of souls, since a particular institute is also against condoms, but considering how that institute is like most things dominated by males, they draw the line when something happens behind a vagina and not from a penis.
Religion does sound like a hilarious bad fiction story when you put it like that doesn't it? If you find this bashing, please feel free to point out in the above what i said that is wrong. All i did was remove the names and made it more technical, if that's the right word for it.
On top of that, I do not suffer from a moral superiority complex and i do therefor not find to be better or worse than others, i find that therefor everyone with the proper guidance and not an indoctrination course that speaks of them doing something "bad" or "good" for that matter should be allowed to decide what they find is the best option for their body. Beyond the heavy religious reasons what are wrong, i do not subscribe to the rather world strange view that majority of the women out there would abort as their primary means of anti-conception, bit of a misnomer but you get my point.