It is far from the norm but an occasional request. From what I hear from my younger dom friends who still session, it is becoming more common than what I experienced. One session that stands out is when a regular client of mine asked if he could bring his wife along. She had discovered that he was having sessions and wanted to understand more about it and know that he was safe. He wanted her to come into the dungeon with us and even participate. She wasn't comfortable with that so I logged her onto my computer so she would have something to do for the next hour. She was very pleasant and open to considering her husband could continue to have sessions and their marriage continue to be a happy one. Surprisingly, I never heard from him again. I've wondered how things went for them.
