Heidi
I don't think you are being irrational. I think the basic fact is that most people who do decide to try for children do have a deep down feeling that they want them, even though there are anxieties etc and its a total leap in the dark. From your post, it sounds like you don't really have that feeling, at least for the moment, and that you are quite happy as you are. Thats fine - you don't need to come up with "reasons" for why you feel like that. You just feel like that, and its fine to feel like that.
The worry is that you may change your mind later in life when you are older and then it may be more difficult to have a child. So I guess you should just keep the whole thing under review and if there is any sniff of your feelings changing, have another think about it before too much time passes. They may well change overnight, as if a switch was flicked, or they may not change one iota. Who knows.
I dont know if I agree with your "reasons" for not having children - I think your feelings are valid without needing to construct reasons.
Yes having kids puts a lot of stress on a relationship, but also brings you very close together too. Speaking personally, my DH and I have proper full blown rows much less now we have a child. We do snap more when we are tired, but we are much better at getting over it quickly and moving on. We haven't had a full on storming out the door row for as long as I remember!
Yes, there is the worry and stress. But you can't love anything or anyone without at the same time having the terror of it or them dying. Its the tragedy of the human condition. I think its better to have the love and then accept the grief, than to not have the love just in order to protect yourself from the grief. And love for children is a very very special and huge kind of love.
Childbirth really isn't so bad. Its one thing to watch it on the TV, from the outside looking in, and another to actually do it. You can't really tell how the woman is experiencing childbirth simply by watching it. And anyway, its something that "just happens" - you don't have to make it happen. Once you are on the bus you can't get off, so to speak - you just deal with it and manage it, because you have to. This makes it easier as you have no choice!
As for seriously disabled children, well that is a risk that everyone takes if you try for children. I've got my 20 week scan tomorrow (DC2) and am hoping and praying it will all be OK. In the vast majority of cases, everything is absolutely fine. But if it is not, we will just have to deal with whatever there is to deal with. But if the choice is to take the risk or otherwise accept not having any/another DC, it's a no brainer for me personally.
Best of luck with whatever you decide to do!