The best way to understand it would be to read the book "She's Not There" by Jennifer Flynn Boyle. It's about a Male-to-Female transsexual (MTF) who underwent a sex change operation. Even though the men who you are speaking of are not undergoing a complete change, She's Not There details how the more feminine Jennifer became, the more she became a new person, the person she'd always wanted to be. However, that person wasn't the person her wife had orginally married. Many transsexuals (if the men undergoing these fem. operations are indeed transsexual) put forward a 'face' that is not truly them; they feel afraid to present themselves as who they are. Just as Jennifer's wife had to reconsider her role in their marriage and the man she had once been married too, the lovers of these men have to decide if their new 'feminized' partner is still the same 'man' they loved. Having such an operation is stressful and puts both partners through many rough changes and decisions. Sometimes, their orginal partners may decide the stress and changes are simply too much to live with and they end the relationship.