Yeah the real reason is probably because nobody would publish a manga like that lol.
But the in-story reason, I think, is that titans seem to be purely killing machines. Remember they don't have boobs either, despite the facts about their origins revealed in the most recent chapters (and even earlier, with the "Dancing Titan").
The only thing that doesn't fit into this is the Female Titan's boobs. Why is this I have no idea. Probably fanservice.
This also reminds me of the last 2 South Park episodes
which were a Game of Thrones periody a series that happens
to be about a big wall protecting humanity from Supernaturals
among many many other things.
Though the did some injustice in portraying the author of a ASOIAF
Butters at least can enjoy SnK in paper and video without
weeners hanging all about.
wherever cookies can't get to me...(as if such a place existed...)
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Re: How convenient is Titan's (lack of) genitalia
Well, it does help a lot in regards to publishing however it can't be said that the lack of organs have not been an important plot point in the manga. Titans don't have organs or reproduce, them lacking any evident form of reproductive organ adds to the mystery of their origin (as far as characters in the manga are concerned). Even if the idea was meant to make the manga easier to publish, the author has made this fit extremely well into the plot and context of the story so far. With the organs not having or needing organs it would barely even make sense for the titans to have such organs. No organs have the implication of no testicles.... And no organs also kinda has the implication of no urethra or reason for a penis.