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The main reason that alternative pairings are degrading to the characters is that it implies their feelings are weak and disposable. If the feelings between two characters are strong then they would not easily fall in love with other characters, which is what alternative pairs imply as they make direct implications about the characters due to the nature of being fan made content. And the same as it was before making a high to low comparison about the strength of the character’s feelings is a negative action, and thus a degrading type of statement. Not all deviation from canon is a negative implication, mind you. Only concept that in some way implies negative things about the topic. In addition, not all interaction between characters is “shipping” interaction. Only specific events have romantic connotations, even in subtext based shows. But, even under the false assumption that all interaction is “shipping” interaction pairing the characters with their non-canon counterparts is still degrading to them as you are comparing the high amount of interaction they have with their canon pair to the low amount they have with the alternative pair, again, being high to low, which makes it degrading.
It also does not matter if you “seriously” like the alternative pair or not, because no matter what you personally feel about the pair you are still making the negative implications about the feelings of the canon pair. Sort of like saying “I was just kidding about you being worthless and disposable, haha, it was just a joke.” The intention/verbal confirmation and logical consequence of the actions do not match.