There would be harsh criticism of a man who "took on" a woman with children and refused to support them in any way
Well I guess it would depend for me what you deem as 'refusing to support in any way'.
I wouldn't want to pay my husband's maintenance no and I don't believe it's my responsibility to do so nor should my income be taken into account, however I certainly don't refuse to support my step children in any way whatsoever.
For example, we split bills 50:50, I don't expect him to pay more for his children, same with food shopping etc... I have often bought them things out of my own pocket, clothes I've seen that they'd like or whatever and I will always help if I can and when I'm needed with things like childcare, school runs or whatever, for his ex too. I feel as though that is sufficient and is me supporting my husband and his children. I don't feel that that needs to extend to supporting them financially through my income.
I do agree that it is not fair to use loopholes like becoming a SAHD, although I do believe that that is not overly common (not exes not paying I know that happens a lot but becoming a SAHD to new family to avoid maintenance) and I definitely agree that step children in the NRP household should not reduce payments, as a PP said you're either responsible for step children or you aren't.
However, I'm not sure there would be an easy way to implement what is fair in each individual scenario unfortunately, I'm sure there individual cases where there may be a case for the SPs income being used (the SAHD being one), but I don't think there's an easy way to implement it on a large scale therefore it's easier for the rule to simply be step parents do not pay.
I also don't think their income should be taken into account in regards to university either, it's all very inconsistent. I think it needs to be a blanket 'no' across all scenarios.