There are a few proto examples from western Baentrash fantasy like The Radiant Knight (do not read) that got picked up where the power fantasy (that was already being deconstructed in novels like A wizard in spite of himself and The man who came too early.) was mixed with dumb horniness.

As the chan culture picked up and became more right wing and misogynistic both here and in Japan, this went from moderately problematic to a death spiral dropping all the cool social history and cultural/psychological examination parts of portal fantasy in favour of having a little world that you get to do free war crimes to but it's good actually.