https://jisho.org/word/%E6%99%AE%E9%80%9A
Futsuu can mean normal as well. But the English translation is not the point in this instance. The protagonist clearly considers "futsuu" to be a positive thing, presumably because it's safe and predictable. She literally says "Normal is the best! Normal is love!" on page 5. She complains about the gay dimension making her feel uncomfortable and uneasy multiple times. She's not happy about any of this.
I don't think we should ignore the text just because the alternative meanings of futsuu make the chapter funnier, or our protagonist not straight. If we have to ignore the work's really obvious text itself to make it funny, maybe the work just was never that funny in the first place. And while it wouldn't be okay to assume the protagonist to be straight by default, so far everything she's said point towards straightness.
You lost me there. I'm not sure what point you're trying to make. I followed your link and found this:
普通
1. general; ordinary; usual
2. normally; generally; usually
3. really (Colloquialism, as 普通に)
4. local train; train that stops at every station
5. Normality (behavior): Normality (also known as normalcy) is the state of being normal. Behaviour can be normal for an individual (intrapersonal normality) when it is consistent with the most common behaviour for that person. "Normal" is also used to describe when someone's behaviour conforms to the most common behaviour in society. Definitions of normality vary by person, time, place, and situation – it changes along with changing societal standards and norms.
All entries, leaving aside 3 and 4 that are unrelated to our matter, include "general" "usual" or "common" in their definition of futsu. Which was my point to begin with. There's no mention anywhere that being common and ordinary must be assumed to be worthwhile or desirable.
When MC says "Normal is the best! Normal is love!" she's not quoting a dictionary or a rulebook. She's not implying that mediocrity is hailed in our world as a great virtue or that the commonplace and the ordinary are generally regarded as the bestest of the best. She's taking a stand, a controversial stand, against certain ideals like "aim for the top" "always do your best" or "be ambitious" that are found so often in manga and anime. There's a Japanese proverb that goes "出る釘は打たれる" or, in English, "The nail that sticks out gets hammered down." This saying pretty much summarizes our MC's philosophy of life: she plans to live her life being careful to never ever stick out. She knows that we (the readers of the manga) won't approve of such weaksauce attitude, but she doesn't mind. She's like "In your face, readers!" as she proclaims her manifesto: "Normal is the best!" etc. etc. You see? That's her shtick.
No wonder she's going bananas now that the world has gone through a reality warp and her carefully cultivated normalcy suddenly makes her stand out like a beacon. ^.^ Her one goal in life was never to make waves, and now all the other girls are looking at her as if she's an alien just out of the flying saucer ...