There's that blanket thing again...
I just now how the Japanese think. I'm actually have Japanese friends and have gone to Japan many times too.
You're not getting the point.
You said, I quote, "Any Japanese would assume a romantic relationship is exclusive unless explicitly said otherwise." You could as well say, "Any Japanese would assume a romantic relationship is something between a man and a woman unless explicitly said otherwise." Yes, a lot of people think like that; and it's a source of much embarrassment for lesbian couples in daily life. A lot of people. A lot. Not all.
Just because a great number of people in a country do something wrong, you cannot claim everyone is like that and issue a blanket condemnation.
Too much headcanon, too little canon.
I guess that's your way if saying you didn't pay enough attention.
Start by working on your understanding of logical thought (and your orthography); then we can talk.