You know, it is always possible that the player is getting set up by Julia..... tested shall we say. If the MC doesn't stray and shows he cares about her, and shows no interest in any other woman, then the game plays out as a victory for the players who just want to stop Cornell from sleeping with Julia. The slightest mistake leads to varying differences in the story. But if your plan is to corrupt her, and just sleep around, then the game happily takes you down that route too, showing consequences for your choices, which you may or may not want. So if Julia sees you only have eyes for her, she stays true. But if she isn't so sure, she considers other options, possibly already considering a future after Cornell's ruse is over, possibly without either of them, as she cant trust them.
I think that the dev has written a game in which the MC character as written may accept being cucked or sharing, but it is ultimately the player's own kinks and whims that dictates the outcome. The player wins if they get to the end of the story and the outcome is what they were hoping for, (hopefully that the MC gets the girl, and Jasmine exposes Cornell's deception to the mother, who cuts him off, leaving it all to Jasmine, and Cornell's plan to get Julia in tatters,.... or maybe Cornell getting the girl, and Jasmine getting a pet till she gets bored, leaving the loser MC with nothing....)
If the player likes the idea of sharing, voyeurism, cuckold, cheating or humiliation,... then it seems all these bases are covered too, but have to be set up by actions and points or they don't happen. Saying something is inevitable, or forced just means you chose something earlier on that opened the possibility for that to happen and I think the key point is at the beginning when you talk to Julia and either admit that you have a kink, or you keep quiet and Julia thinks you don't. I think its always a genius move by a developer to make a choice early on that impacts the game long after you forget having made it.