You're probably right
Its just the circustances with Bianca/Jill that make it feel so fucking evil. She gotta be one of the worst friends ever

.
- Discover she is being cheated on and tell Jill
- You and Jill tell Bianca about it, she feels awful but you both comfort her together
- Flirt with her at the cabin, nearly inserting your peen

- Intense flirting at the party followed by essentially foreplay
As a player, I would probably feel more evil making this choice I think (still doing it tho cause I refuse to cheat on Bella...)
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That's what makes it so good.
What so many games fail at (because it's an exhaustive process) is to create to the positive effects of cheating on your partner. It's essentially the inverse of NTR, and quite a potent rush.
If we didn't have an emotional connection, or if they inserted some moral ambiguity into it, the thrill wouldn't be nearly so intense.
As it stands, it is a tiny bit schizophrenic (Jill fanbois are the most whitebread and straight-laced of chick players), so it doesn't make any sense that they'd start cheating on anyone. But it's well worth the suspension of disbelief since 'rule of sexy'.