Honestly, from a character perspective, this path to Jaye proposing poly makes sense to me. Jaye has a lot of insecurities about MC actually loving her, so I can understand her being more comfortable proposing poly when she's very confident that MC loves her and wants to commit to a long-term relationship with her. A committed couple opening their relationship to a third person is a pretty common way for poly to occur in real life. Don't get me wrong, open relationships of 3+ people deciding to commit to each other is another common way for poly to form, but mostly only when the people involved are overall happy with the open relationship and want to make things more committed. For Jaye specifically I can see why an open relationship might not be a good fit for her.
From a gameplay and player experience perspective though, it's a very different story. If they wanted to make committing to Jaye a prerequisite, I'd have preferred a way to do that while still being honest upfront that I was committing to Jaye, but had feelings for Mal rather than essentially pretending that attraction wasn't there.