I don't think applying real life logic to fetish sex-games is gonna be helpful
The fact is Ethan is the reason this whole cuckolding thing started and Nat was/is slowly warming up to more and more ideas, eventually maybe she will be in full control and have free reign over their relationship and become a fullblown QoS slut, at least on the cuckold sub route.
Whether or not you think that is realistic for her character or for them as a couple is beside the point, it's a game and not a commentary on real life.
Having said that, there are many ways you can write Nat's character to explain this sudden shift in their relationship and also her newfound craving for bbc, which would both make sense and be semi-realistic. Maybe Ethan didn't know Nat as well as he thought he did? Maybe Nat didn't know herself that well...all these sexual taboo activities that Ethan has pushed upon her is revealing a side of herself that she never knew existed.
Regardless of how you wanna write it, it's still the more extreme route of this game and realism is not the main concern of any porn game. However good writing helps bridge that gap.