The answer to this question is bound to be highly subjective, and I don't think there even is a "right" or "wrong" stance on this issue. Of course, barring extreme cases, such as one partner being oppressive or controlling, or so unhealthily dependant on the other that he/she would, I don't know, be unable to live without him.

If you decide to say something on the lines of "anything goes as long as he/she is happy", you are not working under realistic assumptions anymore. Everyone is at least a little selfish, everyone, even in a polygamous relationship, has a "comfort zone" and determines what is okay and not okay for his/her partner to ask. Moreover, everyone has the right to be. Just like Alicorn had the right to decide to set those rules and boundaries with her partner. Pretending that nothing the other person does would "ever" cause disturb and discomfort, and you would be ready to accept it as long as he/she is happy about it is certainly very noble, but not very realistic. In practice, there are things we are okay with, and things we are not comfortable with, and that don't simply, automatically, become acceptable just because we value our loved one's happiness (for instance, in this case, by her own words Alicorn wouldn't be okay with her partner marrying someone else, or, eventually, having kids out of wedlock, because there are certain areas she want to be "just the two of them", something, for lack of a better word, "special", shared "just" between the two of them).

In the end, if I love someone I want them to be happy. Check. I don't wat that happiness to be entirely correlated with my involvement in her life -because, well, in that case we would fall in the previous rather unhealthy scenario-. That said, I don't think there would be anything wrong with desiring that a (hopefully not insignificant) part of the reason she is happy is because of my involvement in her life. After all, we are talking about a couple. Without a gesture, an event, a place, some form of special connection... without having something in common, shared only between the two of you, we wouldn't be talking about a couple. Maybe about a good arrangement for the purpose of sexual satisfaction and possible future reproduction. Otherwise, I could simply take this line of reasoning and bring it to its possible conclusion "I love her, I want her to be happy, I don't particularly care if any of that happiness is correlated to my involvement in her life, and apparently that doesn't seem to be the case -> we should not be together (you could say, since she doesn't mind your presence, either, you could still be an item, but we already established that we don't care at all is any of her happyness is connected to our presence, and we are for all intents and purposes unneccessary, redundant).

Of course, the point here is that in the case of an open relationship, or even a polygamous one, that is not the case, we are not going to the extreme where we say "I don't care how little time she spends with me, I don't care if she prefers to be with someone else rather than here with me, because all that matters to me is her happiness". You might be willing to do the sacrifice, but would a relationship where you never saw her, where no part of her happines was tied to your presence anymore (to the point where it wouldn't even matter if you were there or not) even be callen a "relationship" anymore?

Notice that, once again, that is not the case we are discussion. Reading Alicorn's post on polyhacking, she mentioned rules, boundaries, things that made her unconfortable, little priviledges she might want to have... like the fact that the "primarily" relationship (by her own words, 95% of the whole) is that between her and her partner, or the fact that she eventually wants to marry and requires "exclusive" rights when it comes to progeny, if nothing else, or the fact that she reserves the right (psychologically helpful trick) to stop him from going to see another woman, if she does not feel like it (thought she doesn't feel the need to exercise it).