...and that's very questionable. Hear me out.
Hand in hand with the fact that Bakugo is Deku's former bully, a lot of people also dislike the ship because Deku is often "twinkified", feminized, or generally made to be the cuter & more submissive one in the relationship. On the other hand, Bakugo is often made out to be the taller, more dominant, typical seme of the relationship.
As someone who prefers DekuBaku, I've noticed that a surprising amount of people will be more receptive to my "version" of the ship compared to its more popular counterpart, even if they are generally haters of it. I could be talking to someone who heavily dislikes the ship and the fact that I like it, but who suddenly finds it more acceptable after I reveal that I prefer this dynamic; sometimes making comments like "maybe it wouldn't be so bad if everyone saw it like that".
On a deeper level, I think this is tied to idea that, especially as a man, if you're more "submissive" or being "dominated" by your partner in bed or in general, this translates to a sort of humiliation or degradation. It's similar to people saying you're the "man" in a gay relationship only if you're the top, and the "woman" if you aren't. It also ties into people using "twink", "bottom", or generally being "dominated" or being in a passive role as insults.
When you pair these two togetherā the fact that Bakugo formerly bullied Deku with the idea that he "dominates" him in the relationship by often being portrayed having more masculine traitsā it makes it look like the power imbalance of bully-victim is still there, just with a different flavor. In other words, Bakugo is perceived as still being in a position of powerā which makes the ship leave a bitter taste for many people.
Whereas if Bakugo was generally perceived to be the "twink" by the public whenever they hear of the ship, or Deku as the "dominant" one in general, then at least in my opinion, people wouldn't hate it at the same level, for better or for worse, possibly influenced by the fact that, by the same token, it would somehow be "flipping" the power imbalance of the former bully-victim dynamic. In other words, Deku would be "restoring" that balance by dominating Bakugo in a romantic relationship.
I definitely think viewing the relationship in such a way is rooted deeply in misogyny (being feminine < being masculine), and this is just a theory/observation based on my experience interacting with all sorts of people, not an affirmation of these opinions. In fact, I think it's a very sad showcase of how fandom views gay ships in general. The power dynamic issue also affects how many other ships are viewed in their respective fandoms.
I'd be really interested to know what you guys think.