To me it's not that they can't have a "phase", but to make it the entire game or even a major part of it swings too much towards the cheating side, shows both as weak-willed/hesitant/indecisive and, mostly, just makes for a ton of missed opportunities where a lot of teasing or could have taken place. If "sharing" isn't shared, what's the difference between that, and a game where multiple (unrelated) protagonists have sex with multiple others?
That's already less that optimal. Means they WERE hesitant about it (despite having agreed to share, I presume)? I'll grant it, characters don't have to be always perfect, but it's hugely frustrating when an obvious and simple matter as being honest and forward with your partner isn't an option. They may behave like they're dominant with everyone else, but that will still make me feel like I'm railroaded along the story. At that point, I'd prefer it to be purely kinetic, and that my choices were reduced to "do I want to follow this story, or not".