They ask/care because the pipeline (OTS through FTU; roughly 1.5-2 years) is a lot of work and you have to consider relationships or lack thereof.
If you’re single, that makes the moving, studying, and time apart easier, but the bases are all (for the most part) in dating wastelands where you’ll be pretty isolated. Most of my single bros/broettes in UPT struggled with being single or dated in the circle of other students, which has its own pitfalls.
If you end up with someone below the level of married before going, the AF could care less about him/her and you’ll receive none of the dependent perks. Less pay, no healthcare for them, zero consideration of that person moving/living with you, etc. make things tough to kindle a relationship.
If you’re married, then it’s someone else they’re paying for and pushing along with you. Relationships can strain under the demands of courses, travel/moving, and studying. If they’re with you, time allocation choices have to be made between relationships and studying; if you’re apart, you’ve got all the time available but none of the support.
Kids are a whole other wrench in things.
So, the main point is that they probably want a gauge of what your relationship status is/plans are so they have an idea of the challenges you’ll be facing outside of the pipeline.