It's not saying people shouldn't mention marriage or kids, more that there's a type who do appear defined by it.
Just thinking from my own friends list:
A - Mum to 3 children, 2 have SEND needs and she's a carer for them. Always wanted to be a wife and mum. Never wanted a career. Came out of work to be a SAHP. Does loads of stuff in the community. Her social media is a mix of family stuff and things she's doing voluntarily around millions of appointments and the school run. Never see her going on about her hubster.
B - Dad. Works full time. Posts are a mix of family things (mainly) and a few hobby posts. Wife is SAHP.
C - Dad. Works full time. Wife works part time. Both post family updates. She tends to do more 'ooh look we're married' but it's sweet and they both seem to have other interests.
D - Mum. Works part time. Seems to think social media is her opportunity to tell the world her baby feeding habits. Lots of inspirational quotes about how awesome it is to breast feed and how her baby is happier and healthier for it. Lots of time with boob monster. It's like she's gone from a functioning human adult to a pair of tits with access to facebook.
E - Mum and Dad. Both post family things and things they've done separately. Maybe a date night check in. Fairly standard.
F - Someone who left school with limited GCSEs. Aim in life was to be a mum. Barely had jobs. Popped out babies to her boyfriend really quickly. Moved in. Lots of using social media as jeremy kyle so Simon is the worst fucking dad on earth on day but then they're loved up and baby is having cuddles with daddy the next. She's threatened to leave him multiple times, but a matter of weeks after that there's another baby announcement. On 4 or 5 kids now. Married to him. Most of her posts are about the kids and how hard it is being a mum whilst also laughing about how much of a tip her house is in and how much she loves/hates DH at any given time
G - functioning woman got married and it was like a personality transplant. Everything is about hubby/hubster/#mrandmrs/#marriedlife lots of photos of tea and how the boy did good etc. Lots of inspiration shared quotes about how your hubby is your rock, you love your famalam
Both men and women I'm friends with mention their DP/DW/DH.
Plenty of women have changed their names.
Men and women share family stuff.
Only women on my newsfeed seem to scrap their own personality and do the 'my life is my man and child' stuff.