in my opinion asking people about children is kind of personal, but i don't know the extent of your friendliness with them so I can't make that judgement.

i also want to ask, are you living in a particularly queer-friendly area? are they originally from there? i ask because it could be that they're accustomed to navigating their relationship in an unfriendly environment and so have had to keep their guard up around anything relating to their relationship and sexuality. in that case their reaction would make more sense.

furthermore, it could be that they're used to showing interest (or at least feigning interest) towards customers because the customers seem to like it, but customers never return the interest so they don't expect those kind of questions

just some theories