My Hero Academia is different in this regard, it's MC has a very visible and obvious crush on a girl and responds to female sexual attention (as do multiple other boys and even a couple of the girls though not to the same extent).
Another example of popular shonen with this style going forward is Shokugeki no Soma. You are right that most shounen tend to make their protagonists and usually the other main group of male characters ambiguous in their sexuality (seemingly intentionally). It's also interesting to note that they have no qualms about showing girls fawning over boys (who often are apparently too oblivious to notice their attention).
I find this to be a trope that is not necessarily indicative of some kind of gay bias or something masquerading as ambiguously straight men, it's more of the idea being put forward that males are all socially awkward and inherently dumb when it comes to social interactions. I think it is a result of the types of personalities that many male mangakas have (which is nerdy and awkward/isolated) typically. Many authors work with personality types they have the most direct experience with.
This theory also explains why goku is such a dumbass, oblivious, morally neutral or chaotic depending on perspective. We have numerous interviews with Toriyama, he may be the most interviewed manga author in history, and all of his interviews show some aspects of the aforementioned personality traits. Note that authors of less popular shounen (typically with darker elements) or seinen anime typically do not have this trope present in their work at all and display a wide variety of sexual attitudes and personality traits in their subject matter.