This is no longer a discussion just about a game. You're talking about what exactly you believe the game should do based on your own moral compass & what you think corresponds to faithful or unfaithful when it comes to a partner.Wrong. There was no option to stop V from flirting with River or completely tell him to stop flirting with her. It was possible only seconds before he wanted to bang her.
Me, I wouldn't say flirting with someone is unfaithful to my partner, but that's my belief & hence doesn't bother me in-game. The game doesn't assume that this would or wouldn't bother me, it simply presents me a three-wayed choice later. To assume that it would or wouldn't bother players would be to alienate those on the opposing side of the fence.
Basically, flirting =/= cheating for many people. The game simply can't make rash assumptions. The way the game handles this is an OK middle ground.
Edit: It wouldn't be hard to have an "Already with someone." option, but that's about as far it could go. No specific name-naming, that would require way deeper depth than we currently have.