And your point is that there's absolutely nothing heteronormative in that? Maybe you should start with checking out the meaning of that term.
Also, dynasties have had gay members throughout history.
Usually, they "took one for the team" and it wasn't an issue.
Homophobia as a specific thing has existed at many points in history but far from a consistent time or at different levels. In Davinchi's time, it was a fine-worthy offense and that was considered draconian by some.
my point is that its not worth commenting on given the setting, we see how important marriages are in the setting, especially in the human noble origin
In think Origins handles it pretty well with the Couslands.
They're aware, possibly, their son or daughter is gay but are encouraging them to marry anyway. Still, they don't mind them being in homosexual relationships.
Which is how most of history handled it.
It's not right but it's "believable" (as would, simply, the taboo against homosexuality never existing)