Unless there is a young baby breastfeeding then there is no physical reason why the mum needs to stay home and look after the kids. If the mum does, changes all the nappies etc then that's a rod she's made for herself.
When parents split there are going to be changes. If a dad, previously with a SAHM, is prepared to step up, manage their work hours and share care ....then unless there is a young breastfed baby involved he should be allowed to. There is no reason why a dad can't care for a child just as good as a mum (even if she was a previous SAHM).