There’s a reason interviewers in the US won’t (or shouldn’t) even ask if the candidate has a spouse. If they ask something about that specifically, and the answer indicates some kind of protected status (a man says “my husband” or reveal which place of worship they got married in) and they then decline the candidate, the candidate could make the claim they were denied because they’re gay or practice whatever religion or something else.

Asking personal questions could be seen as a way to elicit information about a protected status and thus give a rejected candidate ammunition for a claim, whether warranted or not.

It’s best to just keep questions focused on the workplace.

Never ever prompt someone to discuss personal relationships in an interview. The moment the conversation drifts into religion, family status, child count, sexuality or gender makeup, or any number of other things, you can easily run afoul of state or federal laws, or open yourself to discrimination lawsuits.

Discrimination of sexual orientation, for example, depending on how it's asked. Just one of those areas best left alone in an interview

Women in a committed relationship can enter a medical situation that renders then unable to work for 6-9 months, + 2 - 3 years of leave afterwards. Men don't, that's just a month or two twice.

It is illegal, and in my book also immoral to deny such a candidate, but the other side of the coin is there.

Employer fishing to see if the person is married, which will require additional dependents on health insurance. Married is possibly more likely to have kids and take more time off for them or maternity/paternity leave.