This is a rather complex and often controversial topic. However, in general, such stories might involve the man realizing he has different attractions or desires that he feels are better met by a shemale. But it's important to note that every situation is unique and these scenarios can be influenced by a variety of factors including personal exploration of one's sexuality and relationship dynamics.
There could be several reasons in such stories. Maybe the straight man has been exploring his own boundaries of attraction and found himself more drawn to the unique qualities that a shemale might possess. It could also be that there were problems in his relationship with the female that made him more open to a different kind of relationship. But this is all very individual - some men might be confused about their own sexuality, while others might be simply looking for something new and different in a relationship.
In these types of stories, one aspect could be that the straight man has been exposed to different sexual identities and cultures which made him question his previous relationship. For example, if he has been in an environment where he got to know shemales more closely, he might have developed feelings. Another factor could be that his relationship with the female was not fulfilling in some ways, like lack of emotional connection or sexual compatibility. However, these are just general ideas and each case would be based on the specific circumstances and the individuals involved.