Maybe it's not as harsh as the f-word but just because some gay men use that for themselves doesn't mean it's not a slur. And "trap" is along the same lines. Some people use it for themselves and that's fine. But that doesn't mean it's ok to call just any trans person that. A lot of people still use that as an insult and for many trans people it will feel hurtful even when it isn't meant to hurt. So, don't try to "convince" anyone that it's not a slur - if they say it's hurtful you should accept that.
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