First off wanting children is OK! My stance here is not an indictment of people who want children. My parents wanted children and they're great! (Plus, you know, thanks for the gift of life and all that)
This isn't a phase
I've known for a long time (close to 10 years) that I don't want children. At this point I can safely say that it's decided.
My undesirable genetic traits
I would never want someone else to go through what I did
Worried that I don't have a long enough attention span
I've never been committed to anything for more than a month. I've never wanted to be committed to anything for more than that.
Worried I would resent them for 'stealing' my focus
Lack of freedom
Having children shrinks a persons worldview
this is only really true while they're a small child
Lack of autonomy (bodily / personal / time)
it's not about my needs anymore
Secondary Weaker Arguments
The world doesn't need more children
Children are expensive
Babies are annoying
The idea that all of a sudden this tiny person I just met would mean more to me than the partner I chose freaks me out
Fears & Concerns
What if I fall for someone who really wants children?