What is sexist, the fact that men and women handle the work/home balance in very different ways, or the company's recognition of that fact?

The company's assumption that the stereotype of how women handle work life balance applies to any specific woman. That's pretty much the definition of sexism. Also, the fact that a company would never ask if a man had any time consumer hobbies or other sorts of things that might prevent them giving 100% to their jobs, because when kids and women aren't involved, companies generally assume that their employees are grownups and can balance their various obligations like grownups.

By that metric, car insurance companies are also sexist - they charge individual men more because stereotypically men are riskier. Of course, that stereotype is based on data, but so is the stereotype that women take more time off when they have children.

Why is it legal to discriminate based on data in some situations but not others? I think the only solution is to remove the logical justification for discrimination: force men and women to take equal parental leave.

I have always wondered if insurance companies had the data to show that people of Asian decent (for example) have more car accidents, could they jack up their insurance premiums?

No. That would be racist. There are all sorts of protected groups. Men is not one of them.

discrimination based on sex for insurance is now unlawful in the UK, as of last year.

as a result car insurance prices for women increased significantly, while mens remained the same.

additionally annuity rates for men dropped, while those for women remained the same (the provider is no longer allowed to factor in that men on average die several years earlier).

That annuity case is batshit-crazy, IMO. Sad that we've gone so far in the chase for fairness that we demand equality in the name of fairness in cases where it makes no sense.

I'm all for fairness. Equal is not fair in all cases, nor is fair equal in all cases...

If I had any other time-consuming hobby and started phoning it in at work, the company could tell me I may be dismissed if I don't shape up. Only parents get the nuclear option, and thankfully most of them don't abuse the possibility very heavily.

That's because parenthood ensures that companies have future customers, while hobbies don't. Facebook stock isn't worth much if all the 13-25 year olds that buy the crap advertisers peddle on it is a shrinking demographic!

Whether employees of a particular company have children or not is irrelevant to demographics on the national scale.