Before reading note the difference between a few key terms:
Stance, and opinion are not the same thing. Opinions are based on non-factual information while a stance is not. Also, interpretation and deduction are not the same. Interpretation is simply the explaining of data, but all not explains are not based on fact. Deduction is the breakdown of events through the use of law or principle.
Key Point Just because you call something incorrect doesnt mean it is. Words alone are not proof. Which is why counter points must be made using deduction and supporting inferences.
When you pair a character with anyone other than their respective canon counterpart you are making direct negative implications about them and the feelings they express in the series they are from and degrade them as a whole. By pairing them with other characters you are stating that the feelings they express in the series are weak or otherwise worthless. This is due to multiple factors, every action works on the same principle of cause and effect, meaning that any statement a third party makes about the characters has implications about them, whether the third party intended for them to be there or not. Remembering two things; implication means the result of logical consequence, and that there are three base states of action positive, negative, and neutral. All action falls under one of those three categories. To degrade something means to make a negative action towards it. Note that you can make degrading implications about anything, not just living people, which is why its factual to say you degrade the characters by making negative implications about them. Also bearing in mind that all fan made work makes direct implications about the characters for another reason. Its fan work, the characters being used are predefined and developed, if you were not making a statement on the characters you would not be making fan work, but original content.
Characters are concepts that exist as set-in-stone entities. Meaning once their story is told we, as third parties viewing the events, posses all available information about them. Granted their story does not continue in a canon sequel. But even then, as new information is given we as a third party have full access to it. Point being that once a story is told it factually exists as that story and no personal view or interpretation can change the events that actually happened. Just because fallible interpretations can be made does not mean all interpretations are valid. Which is why personal taste and opinion is not a factor when determining whether certain content is insulting or degrading to the characters involved. Also, fictional stories are limited concepts set within the parameters of its creators. People are not capable of limitless thought, so neither are the concepts which they make, which is why all questions about fiction have a finite answer set. The answer set may not be one item, but the set is finite.
A character is given worth through its storys development. This is true because if a character is not developed they are considered a blank slate, and a blank slate has no worth. If a character is not developed then no story about it has been told, meaning its not even a character. Characters are abstract concepts and as such their value is also abstract, meaning it is not a numerical value but a conceptual one. Developed in a canon setting gives a character worth, so when that development is discarded for the sake of pairing the character with someone other than their canon counterpart you are implying that said development is worthless in the setting where choose to pair the alternate characters. It makes this implication because in the setting with the alternate pair the canon feelings of said characters are not being used. To not use the feelings is the same as saying the feelings have no worth because if the state of being used does not give them worth, nothing does. Just saying I give the feelings worth because I like them is not the same as actually giving them worth because you just stating something does not make it true. Similar to saying I am running while you are actually walking. Statements alone do not prove fact, the result of logical consequence proves fact as the results can be repeated and tested by multiple parties. Remembering that value comes canon development and implying that canon development is the same fan development is making a high to low comparison, thus is a negative state of action, and is thus degrading to the story and characters. Also, to be degrading does not have to have an actual effect on the content. Degrading is the only the type of statement you are making. It should also be noted that the term canon is used to describe what is factual within a fictional setting. And the one way to prove something is canon is to be able to support it through events in the series. Which is why the not all official content is canon, and some series have multiple canons.
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 characters 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.
To pair the characters with multiple people you are also treating them as interchangeable and disposable, which is highly insulting because it make many negative statements about them; that their feelings feelings are weak, that their character as a person has no real substance, that theyre not capable of being faithful to anything, etc etc. If you claim that any situation you put the characters in will lead to romantic development then thats the same as saying all romance involving them is cheap. Basic math will tell you this. Its a simple 1 to 1 ratio, as delta when exchanged is zero that mean everything is worth because no gain is possible. To simplify it further, you are claiming that there is nothing special about the canon relationship because it could easily be replaced. And of course, implying a strong relationship is nothing special is a high to low comparison, thus it is degrading statement.
In closing; please be aware of the difference between an opinion and a stance. An opinion is based on non-factual information. And all of the things stated here are based on the logical cohesion of events and their results and can be repeated by other parties using the same process, thus they are facts. Also, as stated before, just saying words alone is not proof of deduction, so simply calling something wrong is meaningless unless you support why its wrong. And if something is an opinion rather than a fact, you can prove its an opinion by pointing out the flaw in the logical process where a factor depends solely on a personal value rather than a value determined through the development of events