[X] Sixiang the confidant, the friend, the fun older cousin to her other spirits. Things might not be the same, but that was the path she wanted to walk.
I have never really enjoyed Sixiang as a character, however their narrative purpose is to connect Ling Qi with herself without too much navel-gazing, to make her aware of the instinctive reactions and to force her to re-evaluate. That purpose would be ruined with romance. Sixiang would be a poor lover/partner for Qi, as she would spend more and more time in the Dream with them, and less on the Communication with other people. It has already been mentioned in the story that cultivators that turn into hermits or self-isolate are generally useless to society, which is a view Qi herself agrees with. Sixiang is also far too mutable, I feel. Ling Qi, as the main character, should be the most mutable portion of our story. The entire premise of the story is how she, herself, changes. She requires some form of stability, or at minimum, non-instant change, as liminal spirits are very prone to doing. Humans, though we do change fast, don't change quite that fast.
In terms of story plotline, there are no political or social gains, which is something that, now that it has been introduced, must be considered. Perhaps if Qi had accepted Sixiang the first time, or if the forum had chosen the Sect route, I would find it more reasonable, when there were no other alliances to be made or power struggles to consider. There still might be skill gains, in Ling Qi's realm of Spirit-Speaking and in her role as Priestess, however I believe these are accomplishable without the relationship as well.
I also have a difficult time basing my choices in "but the drama!" or "but the chemistry!" opinions. Chemistry belongs in a science lab, and drama is not a foundation to build a life on, unless one is actively participating in a theatre production.