Hmmm, I understand the dilemma you are in, and I'll try to look at it from an HR viewpoint:
If there is someone above the MD to whom you can go, then I'd suggest that. Again, it's possible that they'll ignore it because of the MD's (apparently) known problem with porn.
I would advise you that for legal reasons you need to be very very careful with the information you have on this guy's workplace transgressions. It should not be taken outside the workplace. It's basically to CYA should the guy get nasty.
However, if you're careful, you could use generalities when speaking to the WTS or other elders.
As for the WTS side of it, I am surprised that Bethel has ignored it. But then, it may be that they choose to place anything coming from an inactive/apostate person as being less reliable than coming from another elder or CO. Or it's the typical old boys' club mentality, and he'll be protected no matter what. Particularly if he hasn't actually committed an actual crime, nor has he acted on his sexual desires.