Its unpleasant to disapprove of another friends relationship.
Jenna was raised in a very conservative home and taught that all LGBT people deserved to burn in hell, and she just found out her best friend is gay.
Jenna wants to reconcile her upbringing with the fact she knows her friend is a good person.
Jenna decides that her parents were wrong and gay people aren't very different from anyone else.
Jenna decides that her friend is just going through a rebellious phase and isn't really gay.
Jenna and her friend become even closer and Jenna is eventually the friend's maid of honor.
Jenna constantly says offensive things about LGBT people in front of her friend, eventually ending the friendship.