When this woman is weirded out by her hosts, she asks Reddit:
I'm currently spending the week visiting a couple that I've been friends with for many years now. I get on really well with both of them and it's nice to catch up and spend some time in their part of the country.
They have a nice house and recently added a private hot tub area out the back and said I was welcome to use it. It's got a small sauna, the hot tub, and an outdoor shower - it's all enclosed for privacy.'
They're fairly open about the fact that they use it naked. I know they've visited European spas where swimwear isn't allowed and stuff. They're not naturists but they just prefer not to wear anything in spas.
They've got some little jokey signs up in the spa area about being nude and another about etiquette like showering before getting in the hot tub and sitting on a towel in the sauna etc but one of the etiquette rules said that swimwear was not allowed and I ignored that.