So many things about this post I dislike!!!
For a start you're obviously one of those sharp-nosed mum's at the school gate who is into everybody's business. I've got literally no idea what hours the mums of my DD's classmates work, unless they are my very close friends - why would I? I also have no idea, if they work part time, how easy it would or wouldn't be for them to simply snap their fingers and "increase their hours" - personally, I had to put at least a year full time in at my workplace before being eligible for the flexible working policy and had to make a strong case for reducing my hours. My youngest is still in nursery and I spend my day off with her; but I don't think that in 3 years when she goes to school I'll simply be able to turn around to my boss and say "so I'm going to increase my hours (and salary) by 20% now because it's convenient for ME, that's cool right?" I can't think of a job or a sector where it would be possible to do that in the current climate, unless they are all NHS clinical staff. So I can only assume your certainty that they could "easily" increase their hours is pulled out of your hat.
For another, how on earth do you know that their husbands/partners don't also work flexibly or part time? Mine does - we both work 0.8FTE and so we can have a day off with our youngest daughter, and so eldest can actually finish school at 3.15 a couple of days a week instead of heading to after school club. How would your nosy self know that unless I'd told you? Incidentally neither of us are looking to increase our hours when youngest is at school. We like the balance, spending more time with our daughters, getting various admin, shopping etc done on days off so weekends can be family time. We have a cleaner too actually (horrors). Because we prioritise our time over what money we can make with it. And yes we might occasionally moan that money is tight, or not send our children on a £££ skiing trip with school so we can afford to do this. But that's because big ticket items aside the lifestyle works best for our whole family.
For another, this sanctimony about "pulling your weight" and not "putting all the pressure" on their poor poor husbands is just ghastly. YOU ARE NOT PRIVY TO THEIR DECISIONMAKING AS A FAMILY OR AS A COUPLE. And unless your FT job is earning exactly the same or more than your DH's FT job, then you are in the same position of not bringing in as much financially to the family pot, whilst not providing the useful cover a PT parent does. Do you think no woman should partner up with a man who earns more than she does and vice versa? What if you start out on level pegging but one of you gets a big promotion - if the other party can't keep pace economically, should she/he separate from their beloved partner in the name of fairness? What is the principle at stake here? Your personal preference for how you and your partner manage your affairs is not a moral good that all couples should aspire to.
Nobody should take the piss in a relationship. But contributions to a family take many forms, not all of them financial. You cannot decide from your observer's standpoint that the women you are talking about "do nothing" whilst heaping pressure on their husbands to earn more. You just don't know. You are assuming. Quite probably out of envy you will NEVER admit.