"So great weather we are having isn't it?"
Seriously, we don't tell people for myriad reasons, this being one of them.
But if they somehow find out we would say something along these lines, "we respect the choices you make and don't judge you, you are free to live your lives as you see fit, and so are we. If you want to discuss the lifestyle because you are curious, we would be glad to talk about it with you in private, if not there is no need to continue this discussion. If my choices make you uncomfortable and you prefer to end our relationship, then I understand. I value your friendship so I hope you don't make that choice."