next time
I met a new to this area guy this past week and invited him over for dinner last night. I am looking to build up my circle of friends to do things with. I had fixed us a meal and then after the tour of my travel themed house I invited Anthony to a hot tub soak. His first reaction was " I wish i had brought a suit" i just laughed and said " no need for those here, in fact I don't allow them " and showed him the posted house rules. He had to get going due to his schedule but promised next time we meet up he would soak and it would be by the rules.
I keep a similar "spa rules" sign posted by the patio door. Usually most visitors are okay with the "no swim suits" rule and ask where they can hang up their clothes. But once in a while I do get a reaction like you had from that guy. I guess it is often better to have that than to have a confrontation or argument later on.