A: Tantra has always been considered a secret esoteric teaching. It was never part of mainstream religion and does not have any codes or moral guidelines that I am aware of. In fact, the Indian mystic Osho called Tantra the "path of Kings" - kings make up the rules. Yoga, on the other hand has many rules, techniques, and restrictions and is referred to as the "path of the warrior." Warriors follow rules.
The Kama Sutra is a moral and philosophical treatise as well as a technical guide to the art of loving, physically and emotionally. It clearly states disapproval of extramarital affairs as well as unions between partners that promise to be difficult for other reasons. Still, the Eastern view at the time (approximately 3000BCE until 1300AD.) was generous in regard to those who broke the rules. In fact, it recognized that people always break the rules and rather than holding rule-breakers in moral contempt, the Kama Sutra offered guidelines for how to handle those situations.
Where does this leave you in regard to your former wife and her relationship to the man with whom she wanted to explore Tantra? I wonder if your negative feelings are toward Tantra or toward your wife? Tantra can be anything people want. There are so many interpretations of Tantra that sometimes I find what people are calling Tantra is unrecognizable by what I have come to understand from my studies and practice.
Rather than taking this situation on as a moral issue, you can look for the lesson in this painful experience. What is it you are feeling in this situation and how are you relating to these feelings? If I were in your shoes, I’d be feeling hurt, disappointed, rejected, abandoned, angry - probably some combination of many emotions. It is tempting to look at the situation as morally wrong, even easier in some ways. You can lick your wounds and feel righteously angry; but as in all relationships, the hurtful situations point us in the direction of discovering deeper places in ourselves. What happens if you acknowledge those feelings in a friendly generous way, allow yourself your hurt, anger and disappointment? After allowing your feelings to surface and be there, see if you can feel loving kindness and compassion toward yourself and your situation.