No; if anything, Natsumi is probably bisexual/demisexual, with a previously stated fondness for people she considers "small and cute". This is a FLASHBACK (and thank you for FINALLY including it!). This flashback (which should be inserted in Volume 9 after Chapter 178, and was already released in the English Seven Seas edition) was intended to give context to the events, present and future, of the story. Now, people seem to have a habit of reading their own biases into situations where they may not exist, especially in this effed-up day and age; so be aware of that, lest you wind up disappointed. Best to just take a story at face value and not have any expectations. Let's examine this and other events in further detail:
Natsumi seems to have had a girl crush on Futaba during high school, due to her being "small and cute". The guy that confessed to her was the high-school track-and-field jock; pretty much the opposite of what she's into. I'm guessing Natsumi is only attracted to a certain specific type of small-and-cute person with whom she develops a close bond, such as Futaba or possibly Yuuto--after all, she tells Yuuto that he and Futaba are the only ones she lets into her apartment (chapter 196). Realize in that same chapter, Natsumi not only allowed Yuuto to stay at her apartment, she even changed into rather provocative clothing for being around a high-school boy (tight-fitting short shorts and a loose-fitting tank top covering about half of her boobs), all the while knowing he has a crush on her. She gave Yuuto an alcohol-aided confession regarding her past with Futaba and her regrets about not yet being "a proper adult" (chapter 197), obviously showing that she trusts him. Not to mention that she finally cuddles up with him on the sofa and falls asleep on his shoulder. Do with all that what you may. In addition, Natsumi later flat-out gives Futaba the shove she needs to realize her own feelings for Takeda (chapters 199 and 204), and seems to strongly support Futaba in that regard, giving Futaba her blessing.
As I'd said, the official Seven Seas English Volume 9 has this inserted after Chapter 178; although it's translated a bit differently (the official English chapter has Natsumi not forgiving herself if Futaba is taken away from her), it still makes more-or-less the same point. I'm sure there will be further developments. Natsumi seems to have considerable affection for Yuuto at the moment, so we'll see where this goes in future chapters.
ALSO: On an unrelated note, once I started buying the actual English manga releases, I found that many of these scanlated chapters have had content clipped out, from partial to entire chapters at a time. It would be great if the current scanlators would go back and fix some of the content that had been clipped out by previous scanlators (Reddit Translations seems to have been especially guilty of this!). I do enjoy reading the scanlations, not only to get a jump start on the released editions, but also that the colors are MUCH more vivid than the paper-print version, especially with darker colors (I'm guessing because they're taken from an online version).