They have already been correctly informed about gender issues. However, they may cared enough to listen. Or, they may strongly believe that transgender persons are wrong for outwardly expressing their gender identification or crossdressing. They are the type of person who disrespects us because they cannot see beyond their own biases and opinions. Basically, they don't respect us because they don't respect themselves.
Some ignorant people deserve to be ignored.
Unfortunately, doing so is not always possible. These people need to be handled, particularly if we interact with such an individual on a regular basis. How you handle them much depends on the situation, although in nearly every circumstance it is wise to avoid allowing the situation to become a question of right or wrong. Like schoolyard bullies they seek out victims, because they believe their opinion is right and can impress it on others. I like to handle ignorant people in a variety of ways.