I actually do think Tolkien's world building on elven marriage matters. It is after all part of the canon/setting you are engaging with. It is one element in the vast set of elements you can draw from, and I do think there is something to integrating disparate elements from disparate works to weave one coherent whole. In any case, if you want to write something about LotR elves and marriage, it really can't hurt to know and acknowledge what Tolkien thought about that subject.
And having engaged with that piece of canon you can in fact discard it for the purpose of your fic.
But I do think this should stand as a conscious choice, with full acknowledgment of a departure from canon. Basically, I don't think canon should be so much ignored as rather... Once you acknowledge what canon is, then you can select from it. Blergh, I don't think I worded this well, but like that. Invalidating pieces of canon for your "fic canon" is entirely valid, but that shouldn't mean canon is casually dismissed out of hand, and the departures should be acknowledged.
This actually becomes important in discussions like this. Canon as a concept is especially important in discussions about the work, because it provides the underlying framework for the discussion - it provides what is even talked about in the first place. So, if we talk about Legolas and his marriage prospectives here, the underlying framework for that discussion is in fact the LotR canon, because that is the only common base for that talk. As such, Anon is right to bring up canon.
And then a response like "Well, I would change that aspect in my fic/quest" is just as correct.