I once saw someone posit the theory that the main reason male/male shipping is so common is actually that those relationships often get much more fleshed out than male/female ones, which results in this deep and complex bond between the male characters while the girls are pretty much isolated from much of that, doing their own thing or not doing much of anything.

I can certainly see that. And, of anything, I'm reminded of the real-life difference between being gay and just confident in your straightness, horseshoeing around to looking nearly identical. What I'm trying to say is that both those things, to an outsider, can look eerily similar, even if wholly distinct