Yep. By choosing neither, the main character loses nothing in the grand scheme of things. Also, since
Wildman Dev is going for a certain level of respectable realism in the story; there is simply no real way that choosing one over the other doesn't make the situation between the two of them worse. The jealously that they already exhibit towards each other, would increase ten fold when you add in bitterness and resentment. At the end of the day we're still talking about women here (one of which is basically still a teenager). The emotions/tension would be thick, and would definitely have a negative effect on everyone else on the ship.
Ariel would have been the perfect choice for a monogamy route. Saying that she's only jealous of her sister is a cop-out, because she was established as the naive romantic type. Whether her sister is involved in the harem or not, there'll never be enough time to satisfy her needs, unless she's willing to be the third wheel and watch every time the other women "have needs". The PC is also already much more interested in Skylar than her anyway, so is Ariel gonna get jealous of Skylar next?
I'd say if players are willing to cut out the majority of the content and go for a monogamy route, just let them. That this possibility is just handwaved away, because "women have needs" puts the level of realism pretty low. Apparently it doesn't matter, if either Haylee or Ariel get upset, so why would it matter for the other women? Nobody commits to a relationship with the expectation to break up, so why does it matter, if they're upset now instead of in the future?
Ariel could offer some much needed variety between all the other LIs, who are basically all kinky, bi-curious and willing to share. I mean, even Nemy jumps on the opportunity to join the harem after drinking a little bit of alcohol, even though it was established that nobody gets drunk on the ship, because the alcohol is rationed. A monogamous relationship would also open up the possibility for cheating and other related kinks.