There was a wonderful post here a few months ago, on a similar subject. The poster said that "if a married couple have one baby, then another, and then another, it means there's less money to be spent on Child 1, with the arrival of each subsequent baby."
And that's true of couples who are separated. If either party goes on to have another baby with a new partner, then there's an extra mouth to feed, and the available money is divided up a little further.
When a couple split up, the finances are determined by the circumstances at the point of the break-up. Life changes, situations shift, and surely the finances can't be set in stone forever? Too often you hear that a man pays such a large amount to his ex, that his 'new' family is almost on the breadline. You can't 'ring-fence' the maintennace that was agreed when the split happened, expecting it never to change, no matter what.
Having said that, if I were a mother who's payments reduced just because my ex's new partner had a baby, then I probably wouldn't be too happy ........ the present arrangements for calcualting maintenance don't appear to suit many people, regardless of which side of the fence they're on.