Personally, this is the sort of thing I never let go. Its appears to be a civil rights violation - those are a big deal.
As an off topic note - lots of houses end up in the wrong hands because when it was bought the house was titled so that their is right to survivorship with someone now pretty much out of the picture. If you get divorced or break up with your significant other - or get married but have your parents on the title because they helped you buy the house - make sure to get the title changed. It is likely to overwrite the intentions of your will.
These two comments are exactly why it is so important to get more information when presented with a situation like this. I am not an expert on the law, but it looks to me that all same sex marriage has been legally recognized as of 2015. I am pretty sure this does not backdate to other ceremonies that were not deemed 'legal' at the time. Otherwise, I would be able to forge some documents that were not required to be signed by any legal authority and claim that George Clooney and I were married back in 2002, and that I should be getting alimony from him.
My best guess is that the salesperson did not know whether the ceremony was performed before or after 2015 (sounds like it was after), and was checking to make sure that the marriage was legally recognized before checking off some box on a form. Perhaps not very tactful, but also not being done with malice.
It is also very possible that he was discriminating against a specific group of people, making it harder for them to get the same service that he affords others. If that is the case, it absolutely cannot be forgotten. Once we are humans allow one group to be considered less worthy than another, we set ourselves up to be discriminated against as well. It is not a world that I feel comfortable living in, so we must stand up against even the smallest amount of discrimination.
I am eagerly awaiting a response from the OP in order to see what the story behind this question is.