Someone has probably already made this comment on the post by @avelera (which I couldn't easily find), but this difference in understanding about what is the closest or most important non-blood relationship clearly comes down to misogyny and patriarchal gender roles. Obviously, the most important person in a woman's life (other than her children) has to be a man: her husband. Women's identities have traditionally been defined relative to their husband's; they change their name, become Mrs. Husband's Name, so of course that's the most important adult relationship for a woman. But does that mean that the most important person in a man's life is his wife? Of course not -- that would imply an equality that doesn't exist. The most important person in a man's life can't be a woman; your most important person should be an equal or a superior, not an inferior. The most important person in a man's life must also be a man -- but for a straight man, of course, that's not going to be a sexual or romantic relationship; it'll be a brotherhood of some kind, either a literal one, a "close as brothers" friendship, or brotherhood-in-arms (a comrade, a work partner).
This difference also corresponds with society's understanding of what a man's vs. a woman's purpose in life or most important activity is. Women are understood to be "made for love" (as Jo complains in the recent movie version of Little Women); their purpose is to find a romantic (and sexual, it is always implied) partner to devote themselves to, then to have children with that person and devote themselves to the children. But even if a man does get married and have children, that's not his purpose; some men might understand their purpose in terms of working to support their family, but many find their purpose in their work, beyond the financial support it may provide. Accordingly, their most important relationship may be with someone (most often a man) with whom they pursue that meaningful work (the brother-in-arms thing: think especially of police partners in crime/detective shows).
These two narratives can converge in fictions where a man and a woman are work partners -- most famously in The X-Files, but also other shows where close colleagues either get together (Brooklyn Nine-Nine, Bones, Castle in the police genre; this also happens a lot in sci-fi/fantasy with people who constantly work together to save the world -- there are examples on Buffy, Angel, Babylon 5, The Expanse) or have a lot of romantic/sexual tension without actually getting together (Broadchurch, arguably Elementary, Janeway and Chakotay on Star Trek: Voyager). In these cases, the woman gets to be the most important person in the man's life as well as vice versa because she has proven herself to be his equal and participates in the work that gives his life meaning.
But of course there are also many (probably more) fictions where a male main character's comrade-in-arms, his closest partner in his purpose-giving work, is also a man. Women (straight or queer) see that relationship -- the most important person in your life, the person you'd sacrifice anything for -- and read it as romantic/sexual, because that's what we've been taught romantic relationships are supposed to be. Gay men read it as romantic/sexual because that's a salient possibility for them, and it's not a huge stretch. But straight cis men often don't see it, because (1) of course another man isn't a romantic/sexual option, and (2) of course another man is going to be the most important person in your life -- but what does that have to do with romance or sex?
Some straight men have become more aware of the possibility of their friendships being interpreted as romantic, which is why they're wary of showing too much overt affection for their friends, or feel compelled to say "no homo" if they do. But the persistence of the traditional dynamic can also explain the otherwise bizarre phenomenon of young men calling each other "gay" if they're seen as too devoted to their female romantic partner, which you would think is the opposite of being gay: it's considered effeminate to regard your romantic partner as the most important person in your life; only women are supposed to do that for their man -- and gay men too, by extension.
stormborns
prguitarman
discoholicmusic
gummygunk
tangentwithtanner
annaxmalina
theartofmadeline
