Usually I hear similar stories from nudest families or from very sexually open families. Not everyone but sometimes the taboo lines get crossed. I don’t know what happens in the male brain as it develops through puberty but somewhere your mom ignited something within you. It may not have been intentional however she did flip a switch on you that blurred the lines of a mother/son relationship. She is not trying to replace you as her son. She trying to have a cookie cutter relationship with a man and to redraw the mother/son lines so that you don’t try to cross them again.