all those characters were exclusively shown in heterosexual relationships.
That’s not exactly evidence against them all being bi
Extraordinary claims demand extraordinary evidence. Bisexuals exist and aren’t always obvious, so “absent evidence to the contrary, that person might be bisexual” is not an extraordinary claim — hell, assuming similar prevalence of bisexuality then as we see now, which is arguably the lower bound given the cultural changes depicted, it’s statistically improbable that there wouldn’t be at least one non-straight person in the main cast.
I don’t care.
I don’t need to know their exact sexual preferences. That is not of any influence to the story. If it was, it would have been in the story and if it was in the story, it wouldn’t have mattered what their preference was, because it would have made sense either way in the context of the story.
Issues of sexuality and gender are already being adressed in this show, there is no need to project them into situations where they’re not.