Marital status is the status of being single, married, divorced or widowed, forming part of one’s personal details.
For women, marital status is also traditionally reflected in personal titles, with “Miss" for unmarried women and “Mrs" for married women. This of course, is somewhat sexist and so some prefer not to follow this usage.
Questioning an adult’s marital status is how we usually ask about their family; we don’t usually ask "how many people are there in your family?" we do ask “Are you married?" to which the natural follow-up question to a positive answer is “Do you have children?"