Most of the time it's not really a big deal, and sometimes they think it's a big deal when it isn't.
I don't see why putting kids ahead of a career is particularly sexist. My kids certainly are more important than my career. It's of course not generally a black and white issue, though. My kids are not hopelessly abandoned I'd I have a career. My career is not hopelessly stunted if I have kids. Nobody's ever goin ro hold a gun to my head and force me to choose. There's going to be tradeoffs, though--sure.
I get what you're maybe implying about staying home with the kids though. That is closer to a binary decision, an closer to killing the career, and usually done by the wife. In fact, mine did. I would probably be warier about disclosing family status if I were female.
Perhaps they actually read their CV? There would be some guesswork involved, for sure, but I think that an experienced recruiter could probably spot the signs at interview.
A few things of the top off my head that would help: Most people list marital status on a CV. Hobbies? If they have lots of them then they're not likely looking after kids. Do they travel a lot. Are they the right age? Does their appearance suggest they spend lots on themselves compared to their income? Is their phone smashed up. Are they up on current affairs? Is their any hesitation when you ask if they'll be able to do late shifts or stay late or travel on short notice ... etc., etc..
You clearly have the presence of mind to laugh this off, but my thoughts trend more towards torches and pitchforks.
you just did that again ;)
Making the assumption that all women will have kids and are mostly likely going to put raising them over their career is incredibly sexist - that goes beyond "not being an equal-opportunity douchebag".. that's like "doesn't understand women and probably has never had a meaningful relationship with one." The woman is best off not working with such an ass imo.