Trap is a slur, especially used often by weebs. Describing gender non-conforming characters who look feminine as "traps", including many canonically non-binary and woman characters, is pretty fucked up when you think about it. To them, "traps" and genderqueer people in general are sex objects, not characters with respectable identities. Most of the weebs that throw that word around are also the ones to do trans erasure, like denying that a character is transgender or otherwise gender non-conforming, instead treating any character implied not to be AFAB as a man; and then often ironically going crazy defending it as "not gay" because that'd be bad – there's a reason "traps aren't gay" is a meme, and it's an unironically defended position by these people. They convince themselves it's not gay by reducing queer people & characters down to sex objects, things to masturbate to, rather than people. If you don't see them as an equal person, it's not gay or immoral, is how they process it. Obviously they won't say that explicitly if you ask them though, they'll just say it's not gay because being attracted to things that look like women is straight or something.

That's why it's used a shit ton in, you know, porn. Not just hentai, but actual real porn. Usually in place of "bitch", "whore", or some other word used to dehumanize women. They're used in the same derogatory manner. It's pretty disturbing when men use "bitch" or "whore" to refer to women and female characters, it's dehumanizing. And it'd be pretty disturbing to well-adjusted people if someone described anyone feminine genderqueer as "a trap", but this is a slur that weebs are fine using amongst themselves.

This problem is made worse by the fact that generic animes started to play into this, that is, they created "trap" tropes (with a lot of objectified/token otokonoko or josoko characters popping up because weebs like it).

You would think those people wouldn't equate anime characters with real people, but this mentality transfers between fiction and nonfiction unfortunately. Often times the way you feel about character identities in media is representative of the way you feel about the identities of real people – just look at the backlash of the gamergate people about the woke "ruining games and movies" by putting minorities and women in them.

Now, I'm not saying everyone who's ever used the word "trap" is a bigot or anything. People use words without realizing the way others see it, and the impact it has. I used it in my weeb phase. But undeniably, "trap" is a derogatory word and a slur used to objectify queer people, and it always has been – it originated in 4channers & internet weirdos getting mad over trans people being at gaming events, posting pictures and labelling them "traps" ("they're trying to trick you into thinking they're a woman to trap you into having sex with them, when they're really not a woman"). It's no different than other slurs for queer people (like "fag" or "sodomite"). It's harmful and shouldn't be used. Persistence on using it shows a lack of respect for (or just plain ignorance of) genderqueer people and their identities.