I started laughing so hard when I read this comment. 
I can only imagine, @jhol613.
Thoughts on "that scene"
Now, THIS is more like the Murata we’ve come to know and love. She does seem to like crafting stories with different social systems. コンビニ人間 had a real-world one, and these two short stories involve more of her own invention. Besides being a little squeamish about what the three young people got up to (just…so much spit in so many places), there were moments that I found quite odd.
First of all, the idea that one of them becomes a mouth, and the other two try to plug up all the holes: I guess it’s like trying to kiss a giant mouth? Not sure what the significance of that is. What got to me was this idea that they were “ravaging” Keita. That’s a pretty violent way of looking at it, though I guess there are those who are into that sort of thing.
Second, words like 赤ん坊, 母親, and other related terms were brought up a lot while they were going to town. I REALLY don’t know how I feel about that. I can’t imagine seeing my sex partner naked, thinking, “wow, they look like a newborn baby,” and still wanting to do anything sex-related. I might have to think all of that over some more. Perplexing.
Finally, did anyone else take note of the fact that they were doing all this for 5 hours?! With such a long time dedicated to it, plus the final lines about how they’re trying to blur the line between their genitalia and the holes of their body (I believe Mayumi calls out the mouth and ears, specifically), it sounds like the goal is to “get off” by touching as few places as possible? Certainly sounds like they want to do it without needing to touch themselves down there. Taking the length of time it took Keita and those last few lines of the section, I wonder if their sex was more of a struggle for him to believe he was enjoying himself, rather that him actually enjoying himself. 
Considering @AmomentOfMusic's point about being underwhelmed
Obviously, everyone finished reading this story already, so my conjectures are mostly just for me. I’ll know soon enough how things turned out (fingers crossed by the end of this week). @NicoleIsEnough mentioned maintaining balance in her comments from this section. I was talking about the reading with my partner (because why wouldn’t you want to talk about this section with someone?) and brought that point up, and he wondered if maybe the issue of keeping things even within the Triple will be the “problem” in the story. Like in the previous week’s reading, Mayumi says three people could move in together: two go to work, and one stays with the baby. But how would they decide who goes and who stays in a fair way? And with her Triple, in particular, she’d be the one to get pregnant, but how would they decide who’s sperm to use?
I’m not sure if this will be what gets explored. There’s only 10 pages left, and that might be a lot to cram into that short a space. I’ll probably have more to say about this depending on how it all ends.
Also, re: the part when Mayumi is talking to her high school friend, and she mentions people in Triples always asking her questions: doesn’t this society still have movies (and porn)? Two people kissing vs. three shouldn’t be all that mysterious. I don’t know, that part bothered me a little.
At least that was my impression.
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At some point I was wondering if I even wanted to keep up looking words because it seemed like 
