I simply don't understand to what kind of people such works are drawn for, maybe the ones who think yuri is a phase and a girl always needs to end with a guy kind.
While I understand your gut reaction and am going to have to brace myself before reading this—it's not at all fair to presume that that's the message here. And I'm going to be a little weirded out if shoujo isekai ever become a platform for preaching that particular brand of bigotry, though of course it's not out of the question and might indeed happen here for all I know (in again with the need to brace myself before reading this XD).
But anyway, my point being, a fully open-minded person would accept that if a bisexual love triangle is indeed possible (which, duh, it is)—either person can win out, not just the gay pairing, nor just the straight one.
Now, as you imply, usually in published fiction the
editors would stop this because the "target audience" doesn't really exist—gay fandoms wanna read gay stuff, most others stick to straight stuff: Tada! Bi erasure.
But I assume this is originally a web-novel, in which case there need be no mystery as to how it happened! Probably someone wrote it as such because they
wanted to, and they didn't
have an editor. In which case it's success or failure, for good or ill, need only be on it's own merits, and it can have whatever plot it wants, for better or for worse.