People who don't understand that there's a difference between transsexuals and crossdressers.
When I've mentioned my last gf (she was MTF) to my family, my dad responded that he "Didn't know much about transvestite culture" or something like that. Later, one of my uncles was confused on what "transsexual" meant until another one used the term "crossdresser" to clarify things.
Seriously, how hard is it to understand? One's a deep-seated identity issue while the other is just something people do for shits and giggles. I fail to see what's so complicated about that.