Traps is basically futa but with a twist.
One explanation I found interesting is that it makes some /d/eviants partially re-experience the thrill of discovering girls again.
In other words, while a regular 2d girl is expected to be attractive, finding the trap attractive makes being attracted to those physical traits regain their novelty.
So if you're looking for a slippery slope OP, it's that you're getting dulled to the novelty and have placed it onto something that should not possess that novelty to repair the appeal. The more you spend time there, the longer the idea that it's supposed to be a male will waver and lose meaning.
As a yuriphile myself, I've experienced this sort of thing and would recommend Beautiful Girls Club for that blurry line you're traipsing through.
http://exhentai.org/g/596949/ff53e037f7/