"If Bucky (or Steve) was a woman, Stucky would be canon".
Yup. I mean, *probably* an actual sexual relationship would still only happen in movie verse because 616 comics verse would still have the age difference issue (in comics fandom, people would be like "what? the love interest is a teenage Mary Sue in booty shorts? And then she gets fridged?). But the number of Stony shippers would quadruple while fans desperately searched for ways to make Stucky not a thing. They'd claim (especially fans who were only familiar with MCU) that Bucky's role in the story was upgraded into a forced canon relationship in order to make Steve look straight and divert attention from the loving glances he's always giving Tony (or Sam but more likely Tony).
They'd say, with bitter sighs, "why can't men and women just be friends sometimes?" or "of course the main male character and the first woman we're introduced to in his life are going to be an endgame couple. Of course." There'd also be a lot of complaining about Bucky existing to further Steve's manpain and/or always needing to be rescued and/or essentially being an object for everyone else to fight over. And the loss of agency and metaphorical rape?
No one would read Stucky because "ewww het".
Sub!Bucky, Omega!Bucky and Stay At Home Spouse/Parent! Bucky would be rejected as "a tired rehash of sexist stereotypes" and...all those fics in which Steve acts scary and controlling and like Bucky owes him a relationship and/or is his property would be quite unpopular. People would be even more livid if Steve was the woman and people were writing sub!Steve or those fics where Bucky acts like Steve can't tie his own shoes without help. ETA or using feminizing nicknames like "babydoll".
If people could have what they wanted but one of the characters had to be female in order for it to happen, they wouldn't want it anymore. What they want is a canon queer relationship and I want that too but...there's a blatant double standard when it comes to how people view relationships when their slash goggles are on.
(on the other hand, the amount of BuckyNat shippers would skyrocket. But only if Natasha was also female, otherwise it'd be "demoting a strong female character to the role of love interest" because she showed emotional vulnerability and sexual attachment to a man).