The employer may believe it is their business (regardless of gender), but that the probability of a man missing work due to children is considerably lower than a woman doing so.
Similarly, employers might ask reserve officers about service obligations, while not bothering to ask them of civilians. They could do this not out of a bias against military folk, but simply because reserve officers have a considerably higher chance of missing work due to service obligations.
The interesting thing here is that as an employer you really don't want to go in to territory like that, because if you end up not accepting the employee for the position you are open to legal action because of having asked such questions during an interview.
Most larger corporations have a pretty long list of stuff that you have to familiarize yourself with before being allowed to interview people independently for exactly that reason. Or they'll have someone from HR breathing down your neck during the interview.
Seriously, these questions are off limits. At all times.
An interview should concentrate on the applicants capabilities, suitability for the job in terms of teamwork, qualifications and so on.
Go outside those boundaries on the asking side and you are in trouble.
If someone volunteers information like that and you use it to make your decision better be ready to back it up with a 'real' reason or you are still going to be open to trouble.
That's one of the reasons why rejection letters are such inane boilerplate, most companies are loathe to tell you why you were rejected for fear of opening themselves up to legal issues.
Asking someone about their service status is done so that you can then choose to not employ that person to minimize your risk for absence, which would lead to people in the service being less likely to get jobs, which in turn would lead people to not want to be in the service because of the effect on their job prospects.
Asking women about their private life (are you pregnant, do you want children, do you have children) would cause professional women to maybe decide they didn't want any children because of the effects that would have on their job prospects, or to simply not apply for jobs. This would negate decades of slow progress in getting the workforce to be reflective of society.
So, by definition it is discriminatory to ask these questions, the effect will be discrimination, even if that is not the stated purpose of the question. An employer could, and many do, for instance hide behind arguments like 'trying to maximize productivity' or 'being worries about the children of the employee', but none of that holds water.
At the end of the day the employer is simply worried that someone is going to miss a couple of days work.
And it's true, there is a real cost to all this, if someone gets pregnant, has to take care of their kids because they're ill or gets called up for the service then the employer will have to find and train a replacement if the absence is long enough, or swallow the loss in productivity for a shorter interruption.
But the cost to that business does not trump the cost to society as a whole if we let people pick and choose on those criteria, that is why it is illegal.
are you pregnant, do you want children, do you have children
But they don't appear to have asked those questions :) (see my other comment)
Personally I do think it is reasonable, though, to ask whether a women plans to have children in the immediate future (say, the next 18 months). It's unavoidable that pregnancy does result in requiring time off (I've heard it argued that a women planning to have children soon and applying for a job is possibly defrauding her employer.... I don't agree with that but it is an interesting way to consider it). They're not going to be missing a few days in the office during a pregnancy - that is long term time off (usually). That's a scientific and genetic fact; asking the question is, surely, fine (and it can be refused to be answered etc.). Discriminating because of it is wrong; but the employer may wish to know so they are aware they will have to find a temporary replacement in the near future :)
I also am not sure why, if your faced with 2 candidates of equal ability you cant choose the one (male or female) who is NOT actively planning to bear children in the next year or so without being marked discriminatory etc.
It sounds like you simply think that forcing employers/unprivileged employees to bear the costs of military service and child rearing is good policy. And it sounds like you would rather label those who disagree "sexist" or "anti-soldier" rather than actually debating this point.
Sorry, but that's just an ad-hominem.
Yes, the employer is worried that someone might miss a few days of work. This worry isn't sexist/anti-soldier, even if it results in fewer women/soldiers being hired. It would only be sexist if fewer women/soldiers (rather than fewer absences) was the goal.
Similarly, employers might ask reserve officers about service obligations, while not bothering to ask them of civilians. They could do this not out of a bias against military folk, but simply because reserve officers have a considerably higher chance of missing work due to service obligations.