That's a common misconception. The truth is actually much, much sillier than that: it's a reference to an old Admiral Ackbar line that would be used as a joke (oh I thought it was a girl at first but it was actually a boy etc) when westerners discovered otoko no ko art (which in itself is a silly Japanese pun). This style of art/character is one where boys look feminine enough to be sexually androgynous and may or may not include crossdressing. It's actually entirely free of implication of sexuality or gender; otoko no ko characters are depicted as liking girls, liking boys, both, crossdressing, not crossdressing, as identifying as a boy or as a girl - there's really no standard to it at all because it's an aesthetic, not something people use as an identity marker of any sort.
So anyway after that joke got so prevalent people then organically began calling otoko no ko characters "traps" and so, naturally, the reverse (a girl who's handsome enough to be sexually androgynous whether crossdressing or not) became known as a "reverse trap". That alone should tell you that it's not actually referring to some notion that the character in question is actually trying to "trap" or deceive anyone; "reverse trap" would make absolutely no sense whatsoever in that context.
The misconception is very easy to make if you're not familiar with the etymology of the term but it's not a slur. It's a goofy-but-now-it's-in-the-lexicon word for an archetype/aesthetic; it's really only used to refer to fictional characters.