So many reasons men do this, depends on so much. Just a few is
she does something for him you dont? she dosent judge him for wanting something? she's new and a different body? He has the control? Sometimes power plays an important part of it, Maybe you turned him down about something or he feels you would think less of him if you knew he wanted something you thought weird? He didnt do it because he loved them, it was just an impluse he couldnt overcome.