I means if the story was about a guy who turned into a girl for a day and being tempted into having lesbian sex with his gf/ jumped by his gf to try out lesbian sex... most would have seen it as random comedy. No one would even think that he is confirmed to be gay because of it
The difference here is that for the hypothetical gender-bender bf, he wouldn't have any change on his/her feelings towards his/her gf and/or sexual attraction. Whereas Kobayashi has never showed actual interest on Tooru until she got turned into a horny dude.
While is indeed played for laughs, is hard to not end with a bad taste after reading the chapter.
I don't know... this is the woman who, first thing after getting drunk and losing her inhibitions, always strips Tooru. I kind of have to agree with a previous poster who suggested that it was simply the presence of something which makes attraction much harder to deny that made her realize that her feelings for Tooru might indeed be something more. Kobayashi just seems incredibly reserved in most situations and so doesn't exactly make her interest well known.