I think this idea that the argument "not all male/female relationships have romantic undertones" being largely met with "of course not, but Mel/Langdon does, and if you want male/female friendships then there are other examples in the Pitt" irks me a little is that I think, personally, that a lot of what people are taking as "romantic" from their relationship is just genuinely the fact that they see each other's needs and accommodate them without question.
There is a very common narrative within a lot of media that says that you will find your person, the person that understands you better than anyone, it is a very prevalent trope. And I think that it's a good trope a lot of the time, of course it's sweet to have two misunderstood characters find each other and feel safe with each other and through their relationship they both get the space to grow as people.
However, the thing for me, is that is almost always relegated to romantic relationships, especially with male/female character relationships. Which perpetuates the narrative that the only person who will see you like that, and who will go out of their way to meet your needs is a romantic partner, that romance needs to be there for that to happen at all. Which both feels transactional, but also a bit too in line with the individualist and hetero-normative societal views of men and women and especially relationships.
That is why Mel and Langdon being friends, good friends, maybe even platonic soul mates, is so important to me. Because showing that you can have your needs known and met by someone that isn't a romantic partner feels important.
I also hate that I have to disclaim this, but I would like to clearly state that genuinely I have nothing against this ship, I do not personally ship it, but I would like to offer my two cents on this character dynamic because I love this character dynamic.