I guess part of what gives NTR its bite specifically in sex games is that if you introduce two characters with superior/inferior dick sizes and stamina, then focus on the sex, it makes sense for the lesser guy to lose.
In real life most girls encounter a guy who at least approaches Arato's size. He shatters her expectation of how good (vaginal) sex can be and then discards her, becoming the seemingly irredeemable asshole ex who she "doesn't know why she stayed with that long".
In real life, she may settle for you because you have romantic qualities that offset your inferiority at sex, or you're a good servant boy with a skilled tongue, or you're just the one who's available. But I for one don't fap to that. Its hotter to see the proper man win out at actual fucking.
That's where I think we landed in the story. The dev is fully capable of writing an ending where Satoshi ends up with someone he clicks with romantically, but I for one would read it, say "daww", then select Arato/Kendo path for what I came here to do.
If I'm wrong and the happy ending is that Satoshi learns to love seeing Mary get fucked better than he ever could, I'd be a happy camper. Buuut I don't think that's what Team Happy Ending has in mind.