It sounds like they thought you were applying for a nonimmigrant B visa, which is for working. In that case they need the marriage certs and birth certs for the dependents' visas. Only reason to ask for those docs, because tourist visa doesn't require them. So maybe the staff just didn't look carefully and assumed, since many people just get the automatic visa on arrival and extend. Just a guess, but the only thing that makes sense from my experience (lived/worked in Thailand and travelled there for visits in between since 1992).

I know the where - that's the usual place. I just keep getting told I need a letter from a workplace requesting the certificate, can't do on my own.