Since we're delving into such places, there are people in my own life who don't want certain labels to be applied to them, such as "him" or "her" and would prefer to go by the term "they", however I know of others who would be very offended if you called them a "they" rather than a "him" or "her".
I know people who call their husband or wife "lover", or "partner" and would be offended if you called out their other half by any other name, and yet I know of others would be offended if you didn't call their their partner by "wife" or "husband".
In family situations two adults does not necessarily equal "mum" and "dad", and can easily be numerous other combinations, and these are all examples direct from my own life.
Despite the fact that I personally know lots of different people who all have different ways they want to be addressed, it's really simple, I start with the pronoun or the terms which I feel are appropriate based on my knowledge of the person or situation, and if I'm corrected I'll use that corrected term (without question) from that point on.
The point is that no matter what you do, you WILL call someone by the wrong pro-noun, or misinterpret a situation. Rather than attempting to come up with a list of inclusive PC terms which for some people simply feel like pandering, a better approach would be to provide sensitivity training, and use terms which invite the customer to give the correct pronoun or situation. They could also easily add in a preferred name, and the ability for people to provide additional information (such as partnership status) between people as part of the booking process / saved in a profile.