I actually love the way you're thinking about this. Here's where to start with statisitcs has sources from everywhere from '05 to '17, though the wiki has info that is way too damn summarized to get a good look at it. When it comes to studies there's actually a lot more Women and LGBT people than you would think. It's kind of boring but the sources at the bottom of the page are some of the few things I could find. (Though I can't find the Tapas demographics from either '16 or '17 where there was some gender statistics but there is some minor stuff about that in the sources)
Firstly, you're right in two regards-- yes demographics are self-reporting but with enough people in a study in a certain area/atmosphere it can show an accurate summation of the actual demographics it depends on how the study is constructed whether or not it is accurate.
Secondly, the self-fulfilling prophecy thing is part of the answer to the equation, though it has less to do with an 'Algorithim' and more to do with advertisement, and in advertisement demographic is the second largest piece of the puzzle at the very least. This is something all businesses do and Tapas is also a business, It's why Cola sells better in certain parts of Asia rather than others. Or how a 12 year old boy can play whatever new shooter is out but once they're sat in front of Civilization 6 they refuse to play it. It's why I used to be able to open a manga volume of Doraemon and read it for hours while I can barely get through a couple pages now.
I don't want to drag this on for much longer, but BL, at it's roots (there are plenty of exceptions but it's an exception, not a rule) is not made for gay men and boys. If it where, there wouldn't need to be a undertone of bisexuality in nearly every damn BL out there. It's a marketing tactic to appeal to girls. It's like Lesbian porn, which is made for men. It has the idealized fantastical elements from the gender of choice slapped together for the viewers enjoyment. Lemme level with you here, it doesn't mean that gay people don't enjoy BL, but there's a set and broader audience. Manga made for gay men never actually got popular, even though it's some of the best drama I've read, it's just not nearly as popular as BL. BL has interesting roots, Yaoi started because female mangaka found it freeing because it was something made for them. It's something that is female dominated because it was started by females who wanted something new, and this is what made the genre popular with the rise of the internet as a whole. Going by that alone we have a good idea of the demographics reading the BL comics already. The Romance demographics are a little bit easier to explain and I'm just not gonna do it justice, but essentially, most of the romance webtoons derive from shoujo which appeals to a more female demographic and there's the fact that Romance has more female consumers in every medium on average unless your genre of choice is Ecchi. Of course I could go into it more but I feel like people will believe I'm breaching over into sexism.
Of course, I could write more, but this has dragged on too long. It's an interesting history to read up on, go ahead and give it a try. I hope at the very least I justified my opinion.