Well tbh, it's not so much that you're leaving her unsatisfied due to lack of trying, it's just her insane libido. To get on the route, you literally just have to agree to an open relationship and decline sex ONE time, at the gym. They still have quite a bit of sex or fooling around, sometimes scenes without variables though. It's just, despite the fun you're having with her, her ridiculous libido has her wanting more.
I really do dislike how it's handled though, I wish it wasn't locked behind declining sex one single time, I'm not sure why that would even matter. That one extra pounding wouldn't make much of a difference with a nympho like her. She would still be craving it regardless. So it should have given the option to get that scene regardless of having sex with her in the gym or not, the only requirement should have been agreeing to an open relationship.
But hey... it is what it is, even when you have the required points, and she doesn't feel fully satisfied, it still shows that she's understanding and doesn't blame the MC at all. She knows he's going through a lot, and expresses as much. So tbh, it didn't feel NTR-ish to me one bit. I mean they agreed to an open relationship, and with the MC's consent she is now starting to fool around with other people.