I think that a lot of the problems with maintenance payments is that they are worked out on a formula basis (no idea what the CSA formula is, as dh's agreement is by coourt order), and thre is no one formula that fits all situations, just as there is no one way that is right to bring up children.
Formulas do not allow for emotions, be they motivated by spite or envy.
I wholeheartedly agree that fathers should maintain their children. I also agree that whatever agreement is in place should not necessarily by adjusted just because another child is born (although I reserve judgement on this, because as has already been pointed out, if another child is born within a family then expenses just get adjsted. They just do.), but where situations change (and this is something that happens as childrne grow up anyway) then renegotiations (either up or down) should be possible.
We could "afford" dd1 when we chose to have her (in the sense that we had no need to even think about changing payments to dh's ex).
Fast forward 3 years, and dd1 needs therapy (she is autistic). This is, obviously a situation that you do not "foresee". If, god forbid, anyhting should ever happen to dss or dsd and they need expensive therapies and occupational therapy to enable them to live a useful and functioning life, then yes, dh's ex would ask for extra contributions. we would, of course be more than willing to pay whatever we could afford (and then a bit more) to ensure that.
We have not even bothered to ask whether dh's ex would consider a reduction in maintenance (which she certainly does not spend on the children, we ended up buying dss 3 pairs of shoes last year (on 3 occasions) as he was going through the teenage growth spurt and was not able to even get his feet into his shoes yet his mother had not bought him any) as we know what her answer would be, and that saddens me.
Yes, she has a piece of paper which says she is entitled to this amoutn of money.
No, it isn't her problem that dd1 is autistic. (dd1 is, however dss and dsd'd sibling)
But I know full well that if the situation were reversed, and it was my ex dh who had had a child who so badly needed help, I wouln't be able to live with myself if I carried on reagrdless, so that my children didn't suffer the horrors of having a holiday less here and there, at the expense of another child's actual enjoyment of simple life (going to the playground, for example, or being able to ask for a drink when thirsty)
sorry for the mammoth post, but I have hopefully shown that it isn't always black and white, and that (the point I started from) it is impossible to calculate these things by simple formula.