If my memory of the first game isn't failing me, the people that are naturally immune to the Z infection have an altered libido which is also related to that ability to boost the healing factor. It's why the girls end up enjoying the zombie rape when it happens even if, on an intellectual level, they're well aware that they shouldn't. It's not that they're weak willed, it's that they literally can't help it.
It's like doing drugs. It doesn't matter how strong your will is, the chemical reaction WILL fuck your brain and fuck your body. This is something similar.
That not so little detail makes both the whole thing with the new protagonist, and the NTR thing, very different than what it'd be if we were talking about average humans. In both cases, the altered libido that comes with their immunity likely plays a part. In the case of the new protagonist, it might even be a necessity. I don't think the vaccine also gives the benefits of natural immunity, so it's entirely possible that the new protagonist is the only one who can work with the girls because of natural immunity and thus, because he has the ability to perform battle healing.
As for why the sudden need for some health-pack things. Possibly a mutation. Mayhaps the new protagonist has different abilities, but needs some sort of booster for the healing.
Or, of course, we're all just overthinking a silly game about zombies that like to murder-fuck people before eating them.