I don't know if I can say much to help but I am sorry your wife is putting you in this situation. If I may say so, I find it a little distressing that she is selling her milk to a single man with no children. There are so many babies in NICU's that really need the milk. She could certainly find a more appropriate outlet for her excess. How did she meet this man?

She is definitely feeding (no pun intended) into the buyers sexual fantasies. My office mate had 2 very premature babies and she nursed them for over a year. She used to sit at her desk and pump her milk several times during the day. There was nothing sexy or attractive about the process.

You have a right to feel betrayed. Your wife and this man are being much too intimate. I don't know what you can say or do but she needs to stop dealing with him and get on with being a wife and mother.