Yes maybe in your feminist utopia everyone would be equal, everyone with kids would be working part time, household tasks would all be split 50/50 and this discussion would be moot.
I don't want everyone to have to do paid work. I think there should be far more recognition of the unpaid work of running a household and raising children, and associated work such as caring for elderly parents or disabled adult sibings.
If someone chooses to be a SAHP, or to take part in part-time or full-time paid work, I want people to be taking those decisions in an informed way. There should be more awareness of the risks and consequences of taking those decisions, particularly for the boring but important stuff like pensions and so on. No one goes into a marriage planning on divorce, but you probably should have contingency plans for one of you not being there. People do die in accidents and from disease, and then the one left needs to deal with having only one income and no chance of maintenance. There have been so many examples in these threads about careers have been stuffed by having been the SAHP - and it's nearly always the mother, that's the point.
None of these choices are made in a vacuum. Most of my contemporaries who have children, it wasn't a free choice for the woman to take time out of her career. Of course women need time out for childbirth and recovery and breastfeeding. But in most of the couples I know, the decision was entirely economic. The woman's pay was often lower - she was often a little younger, so hadn't advanced in her career as much as the man. His pay was usually higher because of that, and certainly in some cases, gender-based pay differences are probably showing up even before children. So the choice is between dropping the higher salary or losing the lower salary, but getting maternity pay instead. It makes it difficult to choose to have the man as the SAHP. And then, the cost of childcare may mean that it's simply not economically feasible for both parents to be in work, regardless of which choices they might want to make. Making these decisions around short term economic necessity then has long term consequences.
Things like the cost of childcare and maternity/paternity leave are things that businesses and governments could choose to improve, but mostly it suits them just the way things are.
That wouldn't stop men who simply refuse to do a fair share of housework or childcare, but it could help make fewer women feel trapped in marriages where they may as well be a single parent as far as the workload goes, but don't feel they can leave because of all the economic factors that have left them so vulnerable financially. (My mother always advised me and my sister to have a running away fund - you don't have to use it if everything is fine, but you shouldn't be trapped because you haven't got one. She didn't explain what to do if you basically live hand to mouth because you're both on NMW jobs and don't have a penny to spare.)
Not having a 50/50 split is a perfectly valid choice, but so many women end up in that situation without ever really having chosen it, just ended up there through circumstances - and it is far more likely to be the woman, which is why it is a feminist issue.