The economics of it isn't just about tax contributed right now. It's about much longer term goals for a productive society. Taking 7 years out of the workforce (for example) will have a significant effect on a woman's earning capacity possibly for the rest of her working life. And it has ongoing effects into retirement.
Even if supporting her back to work is initially loss-making, the intention is that over time people's earning capacity will improve and they'll be able to contribute more. Obviously it doesn't always work out that way, but it certainly won't happen if the person of out of the workforce. And the hope is that they'll be able to build up a pension so they don't need (additional) state support in old age.
A PP gave a really good personal example of how this support allowed her to become a pretty high earner (and net contributor) rather than have any possibility of continuing her previous career slip away over time. There are lots of such examples out there.
It really isn't a prejudice against SAHMs to think about any of this. Nor is it prejudice to say that being a SAHM (for many years often) is a choice that families should bear the cost of themselves (and the ongoing cost in lost earning potential years afterwards).
People aren't necessarily being ignorant or judgemental in feeling that isn't something they want to support through the benefits system for those who can't fund this themselves. It's a perfectly reasonable political position to take. You may want to take a different position, OP, but it doesn't make those who think differently somehow morally deficient.
What is definitely problematic (and outright offensive) is the oft-repeated 'I want to raise my children myself' refrain we see on MN from SAHMs. Mothers who go back to work do raise their children. Fathers (who almost invariably continue to work throughout) also raise their children. People whose children spend all day in school are still raising their children. Childcare workers are providing a service that is quite different to 'raising' those children.