Jan and Monica reportedly met in 1993 when the woman came to Belgium to work as an au-pair for Jan's sister.
"My second marriage was on the rocks at the time and we quickly fell in love," he said.
The couple got a long well, and Monica, 48, was like a "big sister" to his two children from a previous marriage, according to Jan.
"I brought her to Belgium. That was not easy. The Belgian courts had serious doubts about the authenticity of her birth and her identity papers, but eventually they accepted it anyway. I thought she was an attractive woman, all woman. She had no male traits," he explained.
Jan revealed that there were no clues even during moments of intimacy betraying Monica's biological gender, and that his wife had even pretended to menstruate, using sanitary napkins to "to conceal the truth."
It wasn't until recently, after two years of conflict over Monica's full-time job and late night partying, that Jan learned the shocking truth from one of his wife's relatives.
The 64-year-old Belgian reportedly shared the news with his son, who confirmed that he had heard similar rumors. In addition to the revelation, Jan had discovered "amorous messages" from other men on Monica's computer, according to British publication the Telegraph.
"One evening I confronted her and she finally confessed she had been born a boy and had a sex change operation," Jan said, according to the Daily Mail. "My world collapsed in a few seconds. I was horrified. I feel like have been violated for almost 20 years."
The Telegraph reports that the moment of confrontation between Monica and Jan was heated and led to a physical altercation with the police having to be called to their home.
"I pushed her against the wall and said: 'Now I know the truth. Are you a man?' She then announced that she was born as a boy and that she had been operated on. She was now a woman, and so she did not need to tell me about her past as a man. My world collapsed. That evening came to blows," Jan explained.
While awaiting the decision of the courts to annul his marriage, Jan has been denied a request to have Monica forcibly removed from their home.
In the meantime, "We live in separate rooms and rarely speak," he said.