Good lord, i seriously didn't expect a reply after this much time. But seeing that someone else brought it up recently i guess i'm not surprised.
I will admit, when i started this fun two years ago by asking the initial question, i went into it very much wanting you to say "yeah sure lets make romance canon", so when you gave an answer that wasn't a hard "no", i sort of jumped on it. Now, in retrospect, i understand that your answer was far more vague than the yes i treated it as. I want you to understand that i did that out of excitement over the idea of making some kind of impact on the Bionicle lore, something that literally shaped and defined my childhood, no matter how small it was. Now i'm about to go on a bit of a ramble, and im not very good at cutting things down, so i apologise for that.
I've thought about whether or not a Matoran could conceivably learn the whole new emotion of romantic love, and i still think it is entirely possible. You have to consider, what form does a Matorans brain take? Is it a program? A mechanical brain analogue, a little squishy biological brain with electric implants, or even just a purely organic brain? Now i'd see the argument that a program or a relatively ridged mechanical brain wouldn't be able to change enough of develop a whole new emotion based off of nothing but an observation of it, but its conceivable that an organic or partially organic one could over the vast stretches of time that Matoran live for (you could even imagine that some Matoran, maybe Ko-Matoran, would go out of their way to try and feel it, for scholarly reasons).
However, you would also need to consider the fundamental problem; can you even "learn" emotions in the first place? Like, could a human being learn a new emotion? Well, i would argue that often times we do, under multiple different circumstances. Children don't seem to be born with emotions like lust or indeed romantic love, but we develop it, as you said, once it becomes a biological imperative. We only develop feelings like that once we get to the point in life when we need them, so Matoran really have no reason to develop them at all by that logic. However, human beings also develop emotions like jealousy, envy, and irrational anger as they grow, and there really isn't any biological reason for that to happen, it just does. There is no biological reason to be made angry by floor tiles not forming a coherent pattern. Those are feeling that we learn from environmental factor, and through observation. If human beings can do that naturally within a span of maybe 12 years, i think that Matoran, once place in an environment like Spherus Magna in which the majority of the worlds population experiences a set of emotions that stretch far beyond romantic love, could probably learn how to actually feel what they can of that set.
I've been out of this community for a good while now, but even i know that this point of argument, at this point, is pretty dead. However, this is just my opinion and takeaway from what you said back in 2015, and what you are saying now. From the lines you have drawn, i still think its very possible for Matoran to love within the canon, even if the possibility of slim and vague.