What exactly was the emphasis here? That people feel they require perfect similarities in a partner for a successful relationship?
Maybe more this, which would tie into the use of similarities as a tool for partnership. I found the ending a bit surprising, where I thought they'd simply leave the ideology behind and stay together. But they have that extreme notion if one is blind the other has to be as well, which he falls back on and abandons her.
It's basically both of these. The city people force you to find your life partner based on superficial similarity. The forest people take this philosophy to its opposite extreme where relationships are forbidden. David and Weisz fall for each other outside both these institutions (their superficial similarity is what brings them together but isn't really why they fall in love). But the ending shows that they really don't know much about connection beyond superficial similarity. I also thought it was a subversion of the romantic trope of lovers succumbing to the same fate - they either live happily ever after or both die in each others' arms.