So how the NTR works here (and in the Kansen series in general)? You have a gf, and they get randomly raped by the sex zombies, is that it? Any buildup, corruption?
It kinda differs from game to game.
When it comes to the zombies it doesn't really feel like proper NTR but I'll get those out of the way first:
Only counting each route's love interest and characters that were established to be in relationships already most of the time NTR is linked to bad/tragic ends and there's a few patterns:
-She gets infected without being gangraped and the infection slowly turns her, can lead to NTR either from her raping someone else, or having sex with other infected. It's like corruption but fast and without breaking suspension of disbelief.
-She gets gangraped and immediately turns, usually the MC gets infected by her afterwards or dies (most common bad end).
-Everyone is infected early on and starts doing typical rape zombie things, you could count this as NTR
If you include heroines that aren't the love interest for a particular route as NTR then it's mostly:
-She gets raped by uninfected people, usually followed by getting infected leading into the next one
-She gets infected while separated and is fully turned (after having lots of zombie sex) when the MC meets her again
Again, a lot of this stuff didn't really feel like NTR to me when playing it since it's more tragic than anything else and rarely written to elicit feelings of jealousy or whatever.
There are a few instances tho where regular NTR elements emerge:
-MC failing to confess to a girl and she appears to get stolen by another guy
-Girl is only interested in the MC and another guy gets cucked, may have the other guy rape the girl in some routes.
At the end of the day you'll probably find anything and everything if you dig through routes and choices, including very vanilla stuff with a happy end and actual NTR written to cater to that fetish (Kansen 2, 3 and 5 more than others).
Overall though when it comes to "dark" fetishes I would say Kansen is usually more about rape than NTR.