That could've been done if they just turned her into a futa rather than 100% male.
Literally why, futa would work just fine.
Because everyone knows two women can't actually have sex. A man needs to be involved somewhere in the equation.
Seriously, the ending of that actually made me think less of Warugaki as a writer. It completely undermined any impact the story had on me. It had been super dramatic and suspenseful, and then ended with a pointless, fanservicing anticlimax.
It reminds me of that one dumb gag in Maid Dragon. Although, at least there nothing happened and it ended up being a throwaway. (It would have been much better if, say, Tooru had said something like she loved Kobayashi as a woman... But it's a comedy manga, so I guess that's too much to ask.)