Romance is bg by default. No need to complicate things. GL and BL are niches, so they need to be separated from the normal romance, hence the new genres. Otherwise, romance itself will be a meaningless genre.
Then exclude the tags related to bisexual protagonists and multiple protagonists from your search as well. Should solve your issue.
Sorry, but Romance is not BG by default.
"a book or movie dealing with love in a sentimental or idealized way." (Google Defitinion)
"a
story about
love" (Cambridge)
"(1)
: a medieval tale based on legend, chivalric love and adventure, or the supernatural
(2)
: a prose narrative treating imaginary characters involved in events remote in time or place and usually heroic, adventurous, or mysterious
(3)
: a love story especially in the form of a novel" (Merriam-Webster)
"a novel, movie, or genre of popular fiction in which characters fall in love or begin a romantic relationship (often used attributively):We knew it was a romance, so we were expecting a happy ending.Romance novels are popular escapist entertainment.
a novel or other prose narrative depicting heroic or marvelous deeds, pageantry,
romantic exploits, etc., usually in a historical or imaginary setting.
the colorful world, life, or conditions depicted in such tales." (Dictionary.com)
Romance = story about love.
Comes with its own set of tropes and cliches and story beats.
Excluding bi or multiple protagonists ignores the issue of wanting to find
BG specifically or to exclude it specifically. What do bi people have to do with it? What if I want to read about romance between two AIs without gender/sex? About alien creatures with 20 genders? About non-binary people? About girl romancing a Sim in a video game? About a boy romancing a planet that takes shape of a dinosaur most of the time and a flower only sometimes?
Should we create a special sub-genre tag for every instance like that? Why not just create a huge genre tag of BG and just filter THAT out to actually be able find everything that falls outside it?
I'm sorry but we have separate fandom tags for:
僕のヒーローアカデミア
My Hero Academia
My hero academia
Boku no my hero academia
Boku no Hiro Academia
Boku no Hero Academia
MHA
and probably others which causes people to kinda wander in all of them because they're so incoherently compartmentalized. But the overarching category of BG that actually gets rid of most complications and genre confusion is
too utopic?
That's the author's fault for misusing the genres. Not Scribbly's fault.
BL Genre has nothing to do with tropes, it has to do with homosexual romance between men.
Bara has to do with homosexual romance between men.
Yaoi has to do with homosexual romance between men.
BL has to do with homosexual romance between men.
Shounen Ai as a used term has to do with homosexual romance between men.
Slash has to do with homosexual romance between men.
LGBTQ+ m/m has to do with homosexual romance between men.
Tanbi/danmei has to do with homosexual romance between men.
Some of these subgenres have kinda bad relationships with others. As a person who is somewhat educated with tropes of each of them, it's often super hard to find anything on SH because it makes no distinction between any of these.
BL and
LGBTQ+ m/m subgenre tropes are ones found most commonly on this site, and even between these two, you have no idea which of these two subgenre tropes you are going to meet in the book you pick on SH.
And LGBTQ+ m/m and BL actually do have conflicting tropes, so a lot of people in LGBTQ+ community are actually turned off from SH because they think it's all BL since that is officially claimed as the ONLY genre tag for the target audience. (Yes, even without giving it a chance to see that a lot of "what gets called BL here" incorporates LGBTQ+ tropes instead).
And your definition is also missing these:
Homoromantic relationships between men,
Homoerotic relationships between men,
And lastly, ~normalization that writes about couples assumed to be m/m but makes
zero focus on what you called "Homosexual romance between men".
Now, I struggle with this personally because I actually find both BL and LGBTQ+ m/m genre tropes too restrictive -- but I would not demand any of these subgenres to be recognized by SH for now. In the future, it might benefit from AT LEAST the BL/LGBTQ+ distinction. But to say that
BL Genre (subgenre, actually) has nothing to do with tropes is just...

.
"BL -- common term used by the publishing industry to categorize works focusing on male/male relationships
marketed at women" (Urban Dictionary)
"BL is a
genre of fictional media originating in Japan that features
homoerotic relationships between male characters. It is typically
created by women for women and is distinct from homoerotic media marketed to male audiences" (Wikipedia)
"Boys' love (BL) is the common term used by the publishing industry to categorize works focusing on male/male relationships
marketed at women. " (Yaoifandom.com)
And also, just for the sake of it,
"1: dialectal, England
: SOUTHERNWOOD
2
: WORMWOOD" (Merriam-Webster definition of
Boys' Love)

And one I stand by, personally!
__________________________________________________
Sorry got so long, but, simply put: all of what I said above stands for GL to some degree as well and for BG.
For BG it stands even in a wider sence.
A lot of BG might not be Romance. I know I might be a weirdo, but I almost exclusively write stories that feature heteroromanticism and heteroeroticism where there is no sexual or sometimes even romantic development between B and G. I WANT to read BG that excludes Romance like I want to read B+B that does not focus or excludes romance as well.
How do I find a book that has a B and a G as main characters but who never think/act/want to bone each other, yet some of their behaviors might be considered heteroromantic by the reader for the sake of tension? (Think Frodo and Sam from LotR. I want to read
that but about B and G. And I want it to not even pose the question of sex between them). How do I and others find such a book?
By there being a BG that excludes Romance tag, perhaps?
Sorry -- but I literally don't see how BG = Romance. They can be separate genres, for all I understand about either B&G main characters and Romance as its own genre.