[hmr]

I have a slightly different take on this. In your original post you said something about his mother havind "dabbled in.... well" . Some people have a much better relationship with their mother than others and may have had suspicions that this may be something she had an interest in. For all you know he may have suspected she was a swinger and this was a way of getting the info from her. Plus, he apparently suspected that this would not have caused an issue wiith her, which from your post it did not, so no harm.
Although my mother does not know about our lifestyle, I suspect she would not be totally shocked or upset by the information. She would never have gone here, but we have discussed some pretty interesting things over time and I would expect it to be a nonjudgemental conversation.
JMHO
Mr