It's not sexist necessarily, 99.9% of posts will be from mums who are the main care giver about their husband who works full time on mumsnet so fairly safe assumption to make, although clearly not full proof.
I felt the same - that my husband would try to take over and boss me around when I was in the first few months of maternity leave and really, I think that's because he wanted to be involved and had done his own research and had his own ideas of what to do. I felt put out, he felt left out, neither of us were rested and we were both so grumpy and knackered that it was a perfect environment for relationship breakdown.
We both decided to take a step back and I, as the main care giver on mat leave, had to make the decision to allow my husband to make some of the decisions and for me to go along with them unless there was a really good reason not to. Sometimes trying something was all I needed to do to show I trusted his opinion. If it didn't work in the way he was hoping, she didn't sleep or whatever, well fine, back to the drawing board. And if it did, then great. Win win.
It's such a tough time, be nice to each other and cut her some slack. If she's given birth 5.months ago and is back at work already (if you're the main care giver) she's probably struggling with guilt or social expectations about motherhood, and then she comes home and feels left out of the child raising too.