I'm probably not doing this quite the way you described...but, what I have done (to date anyway) is tell him basically what you have stated (not quite that way, but in my own way pretty close). At this point I'm not going to police his activity....unless I feel it necessary to find out if someone else is involved at a future date KWIM?
My gut says there's not someone else and that this activity is simply a turnon for him.....so right now I'm trying to HOM and call him on the behavior. I'm sure I could use suggestions as to how to handle it...so I'll certainly keep your 180 in mind and think about it.....thanks!