I'm not sure I quite understand the complaints about stay-at-home mothers not getting maternity leave. To take "leave", I believe you actually have to have a job to take leave from. I think maternity leave is meant to be a payment to tide a new mother over while she's not working. I THINK so. I'm pretty sure that if you don't go to work before you have a baby, continuing not to go to work is not actually "taking leave". But some are complaining that stay-at-home mums are not going to get any maternity leave from the jobs they don't have.
It seems a bit like complaining that you're being discriminated against by being denied workers' compensation payments from a company you don't work for as a result of an injury suffered on a snorkelling holiday.