@KazumaLee94 For what it's worth I agree with your second comment, if Koto was a guy that last chapter would've been completely different, not to mention her entire character as far as manga-stereotypes are concerned. I didn't enjoy that last chapter very much, it was cheap fanservice without any actual substance, and it says a lot more about Koto's character than anyone here seems willing to admit.
With that said, I still 100% agree with Darklight99's notion that you're severely overreacting. Considering Koto is played as a ecchi-comedy-relief character in most of her appearances that chapter need to be at best taken with a pinch of salt, or just ignored for what it is. Girls pretend-I'm-him dating for "experience" is a common shoujo-trope, even if this dial was turned up to 11 due to Koto's character. Koto-Nanako ain't happening in this universe, no matter how rape-y Koto was. Nanako doesn't have those feelings, no matter how reluctant she was to push Koto away (which is itself a very heavy shoujo-trope, further enforcing that it was just a lame service-chapter).
Misunderstand me correctly, I don't think that chapter gave anything to the story, the comedy or the characters in any way whatsoever. I just absolutely think it can be ignored as Koto-nee being what her character-trope forces her to be and Nanako being what the story-trope forces her to be in such a situation.