God, I don't know what to do. I honestly love that woman. I can understand men wanting her, she's beautiful. But, can't be married to someone like that. I'm going to go out on a limb and say that I don't think this is what modern marriages are all about at all.
Modern marriages are about communication, respect, trust, and partnership.
Yes, there are couples who choose to have lovers other than their spouse, but they negotiate that ahead of time, communicate their expectations clearly, and always respect the boundaries of their partners. These relationships, called polyamorous, do not develop under secrecy, and are not carried on without the full consent of both partners. That clearly is not happening here. Not only is she violating your trust, and disrespecting your wishes, she is putting you at risk of a wide variety of sexually transmitted diseases. This does not communicate to me that she respects you, and your vision of what marriage is for you.