I was genuinely afraid that in the end she would go "GOT YOU" and be really a bitch, but this gets lovey-dovey feelings at some point.
Still sleeping with other guys to get his attention is quite the stupid move.
While that's a possibility we barely know anything about the girlfriend. This is way way more NTR (as it starts with the husband) and cheating (wife's POV) netori on this is pretty much disregarded. Also, I believe when someone likes a tag enough to follow it, they want that, not a 'little bit' of it in a weak kinda of way.
This is completely NOT netori. Netori is stealing someone's lover, but from the POV of the stealer, in this case, not the wife or the husband, but from Yamaguchi, what doesn't happen.