Look, many men (and employers too) have to have it spelt out to them that pregnancy/birth/breastfeeding does drain a woman's physical resources a bit. Maternity leave reinforces this so that women get a chance to take it easy in the last few weeks, mostly for the sake of the baby, and to prepare for the endurance test that labour/birth can be. Some ladies might risk doing permanent damage to their bodies if the keep working too long in late pregnancy.
Some men would selfishly take the chance to be the one to stay home and pack the mother of their child back off to work with barely healed stitches, leaky boobs and troublesome piles. Some men, in a relationship where both parents work, will not take turns at getting up in the night when babies/children wake. Some men take every opportunity for a weekend lie-in or mid afternoon nap, leaving their other half to yawn her way through to bed-time.
Wasn't child benefit originally payable to the mother of the child to ensure it stood the maximum chance of being used for the benefit of the child in question, rather than just increasing the spending power of the father ? (men definately controlled the majority of families' finances when it was introduced).
Please note I am not generalising here but there are thoughtless/selfish husbands and partners in all walks of life and for their wives/partners non-negotiable ringfenced maternity leave is a godsend. Lets not consider it sexist, it just makes best practice in caring for a pregnant woman THE LAW.
Maternity leave is not a perk based on your sex it is necessary to protect pregnant women and mothers of tiny babies from unfair demands of employers and in some cases husbands/partners.
Its not sexist, its a sad necessity in the current government backing of the "get out to work if at all possible" theory of workforce management coupled with potential male attributes of thoughtlessness/ immaturity/selfishness not being altered as the females are by pregnancy hormones.
P.S. I took vey short maternity leave myself for (selfish) personal career reasons but that was my choice.