This is just my take about the manga, but I think there's supposed to be something different with trans boys in this universe and that's adding to the confusion on the whole situation.
Maybe they will realize it is just as you say in the long run, but I think that would be less fun.
Not only do I think Yuuki was originally female I think Rinko's dad was as well. Something seemed off in chapter 1 with some of his mannerisms, how he reacted/over reacted, and kinda seemed like he knew Rinko's libido would increase.