I think you are right. The way the interest system works is if you provide positive responses or flirt, interest levels rise and you will get further scenes (whatever those may be). A simple "I'm not into that" response to Jenny/Grace at the bar could swing those interest levels one way or the other. Everyone is informed, everyone is consenting (or not) and there is no further conflict.I don't necessarily have a huge issue with the way Jenny and Grace's situation is revealed — although it would've been nice for one of them to mention it earlier — but too few VNs give the player the opportunity to back out of a path after similar reveals.
I think the simple option to say "that's great for you two, but I'm not really into that" would go a long way.
Having multiple on/off routes would also help, so if you were playing the strictly Jenny path, and ended up not wanting to move forward, there should be further opportunities to get back on other LI paths.
Everyone is making good points, and I am weighing them carefully. Jenny and Grace have a long history together, a very strong bond, and they're FWBs. This isn't going to change. But what can change is the MC's knowledge of it and whether or not he wants to continue pursuing it. It is too early in their relationship for MC to ask Jenny to be exclusive.
Again, not to spoil anything, but as these relationships get deeper, exclusivity will play a part, which could provide for some meaningful conversations and decisions for all of the characters later down the line.