Just for the sake of example, X-Men's Ice Man, Since he was originally introduced, has always been known as quite the ladies man. Until recently when it was decided that he needed to be gay because that's a current hot button issue, politically. Many fans of his were shocked by this recent alteration to who this character is, and don't understand why this change needed to be made instead of say, just introducing a new, unknown character who happens to be gay.
And before anyone accuses me of being racist, or anti gay or what have you, just think about some of your favorite characters from whatever form of media you like, and consider how you would feel if they were changed in ways that were contrary to how that character has been portrayed to you up until that point.