IS, I feel your pain and we have something in common. In my relationship, W had a gf for a couple of years, but I not only allowed it, but encouraged it. Remember for many men it is a turn on anyway. I understand the fear of losing your W but it is somehow less threatening to me when it is a same sex partner. Bear in mind that unless you have extreme surgery, you cannot offer what the OW can. :-)
After a couple of years, it burnt itself out and the gf was dropped like a hot brick. Part of the reason was because she thought it was going to stick a pin in me! The only reason it bothered me was the way she so rudely cut her off one day and that was that. I have stayed in touch with her myself and been able to support her when her marriage went through a bad patch.
It maybe too far gone now for such an extreme 180, but I wonder if you could steel yourself to take an interest and maybe comment on this OW in a positive and "Actually, thinking about it, the thought of you doing that is quite exciting, tell me more about it" way that will astonish her?. I can understand how it would tear you apart, but just maybe the OW is forbidden fruit, because she knows you feel it is unacceptable and will be so traumatic for you.
