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.