The phrase.
It's hard to tell whether its ignorance or joking, most of the time.
For those uncertain with the slang phrase, a "trap" is a biologically male character who passes as a girl or who can be mistaken for one. The opposite of that would be a "reverse trap". Basically it references to being mislead about a persons gender.
I believe it came from Star Wars, Admiral Ackbar saying it in Return of the Jedi, and 4chan came up with the connection a few years back; /b/ maybe?
I remember back when Hourou Musuko first aired. People had a vague interest in it, and a lot of the comments were of the "Lolololo, boring shota trap anime!" kind, and ignored basically everything the series was about.
I don't have much of a thought on the word, but I'm only fine with it if its referring to crossdressing. Using it to refer to a, say, transperson means you don't think of them as their gender. It's rude and ignorant. But a lot of people have this sort of viewpoint..