It's a porn game, not Hamlet. If dev occupied himself with creating complex personalities, we wouldn't get first sex scene till chapter 50. Everyone in the game has a primitive personality and it's expected given the context plus it gives room for imagination.
Nothing wrong here, woman has her needs.
She should've been honest. It would be better for both of them in the long run to seriously discuss it and either reach consensus or break off the dysfunctional marriage, but she didn't want to hurt her husband's feelings. Tom is one weird chap though in how he tries to limit and control his wife who he claims to love so much. So while I agree that Devin made a mistake by not being honest, calling her 'crappy' or 'arrogant' for caring about her husband's feelings is not it.
"Validate her cheating" part is completely made up and inaccurate, Devin is perfectly fine with what she does. She corrupts gf because it's convenient for narrative. Theme of this game is gf sharing, but since dev decided to make gf 'innocent' and indecisive, she needs to be eased into it by someone else. MC wants to share her and players can't wait for her to do something, but gf can't suddenly hoe it up on her own after pussyfooting for so long, since it's not only unrealistic but will ruin this 'innocent' persona. Considering all the context, Devin does the right thing. And even without context this gf is too closeted, she should take better care of maintaining her pussy in healthy condition. Going without dick for so long will earn her pussy cancer, all sorts of pussy bugs and other undesirable entities. Devin is doing god's work here, fucking opus dei!