My reasons are quite different digerd. Mainly I just don't have the desire for them. I used to think that maybe if I met "the one" and he desperately wanted kids then I might agree to have one if he was willing to deal with the day to day reality of kids/pay for a nanny. So if anything, as I've gotten older I've gotten more firm in my view that I don't want children.
Here are my reasons besides "I just don't want them":
If I don't average at least 8 hours sleep a night over the course of a week I'm a zombie - whenever I've had a job that I needed to get up before 7 for I'd end up sleeping most of Saturday.
I'm really sensitive to high pitched noises which means the sound of a baby crying gives me a headache in less than a minute even if that baby is on the other side of e.g. Primark (as a guide for room size).
I can barely manage my own life sometimes (depression and anxiety) let alone someone else's.
The rigmarole of not being able to go to the corner shop without taking the baby would likely result in me barely leaving the house which would be terrible for my mental health.
The whole idea of pregnancy and birth terrifies me even without the long term effects that a lot women have afterwards e.g. urinary incontinence.
If my relationship with my DP broke down I wouldn't want to tied to him for life through children.
I'd be constantly terrified of DP leaving me and me ending up a single mother like my mum - she didn't do well and I'm very similar in personality to her.
Once you have one you're stuck with it even if it ruins your life.
Having said all of that, I concede that there is a chance that I would love being a mother. I'm not willing to take that chance though because there's a high likelihood that it would not only make me miserable, but some poor innocent too.