I did this once for a massage at a nice villa I was invited to stay at, and the woman doing the massage commented on it to say it made her job easier, which made me wonder if the guys getting massages left their board shorts on.
I’ve found pretty much every spa and hot springs in Portland is ok with thongs. Every time I ask a receptionist they say it’s totally fine for both men and women.
I’ve had the same experiences with wearing a thong during a massage. Have had a couple different therapists mention it’s actually better/easier. They can do better glute work without having to worry so much on having their draping stay in perfect place.
I’ve been going to the same spa for over five years. When I was searching for a place that did deep tissue and various types of therapeutic massage, I asked them if clothing was optional or underwear only. I’m fortunate mine is clothing optional but I wear a thong for some modesty. I also interview every massage therapist and let them know my fitness lifestyle and any pain points or areas I don’t feel comfortable. They have been 100% accommodating to my needs and I now have one therapist that works on me. Anyone that goes to a spa for therapy needs to take as much time and effort into finding a reputable establishment along with a therapist that suits your needs.