I've seen a few times on here where people will talk about het relationships between queer-coded characters (as in, not even canonically queer characters) as "it's important to remember they're also queer relationships!" and as these progressive things when in fact those relationships were specifically because they either couldn't put them in a same-gender relationship, or sometimes even designed as an attempt to derail or divert attention away from a queer relationship/reading that was popular with fans that the creator(s) didn't want to write for whatever reasons. The ones I've seen this the most with -- largely thanks to fandoms I've been in -- are Lupin/Tonks in Harry Potter (a particularly egregious example of the latter) and Jezri in DS9 (which is more the former, though Julian had subtext with a male character whom there was supposedly a producer mandate not to give him as many scenes with as a result), but I'm sure there are many others. In both those cases especially, but in many others as well, it's particularly frustrating because it results in people not only giving creators credit for doing something they didn't actually do, that the fandom did (read those characters as queer), but that those actions were often specifically in opposition to that fandom reading. It's a regressive, anti-queer thing being reinterpreted as its opposite.
And I think where people are getting things wrong here is confusing how you talk about real bisexual people with how you talk about fictional characters. Yes, when talking about real people, you shouldn't act like they're less bi for being in an opposite-gender relationship; you shouldn't scrutinize the reasons behind their relationship choices; and so on. Real people's private relationship situations are none of your business, and no one should feel like they have to make decisions about love and sexuality, about who they date or their personal identities, in order to fit in with what other people think is progressive or queer "enough." Obviously.
But critiquing the decisions made about fictional relationships is different because these were not decisions that were made by individuals about their own relationships, but by creators about their fictional creations. And fictional characters don't have internal struggles and feelings. They're ultimately the constructs of totally different people. And those decisions are not made in a vacuum, but by people in a society and an industry and media environment that have certain preferences for the relationships they want to see in media over others, especially when we're talking about works from the 1990s and the 2000s (Jezri and Lupin/Tonks respectively).
And this is all the more the case when we're not even talking about canonically bi characters! Tbc, choices about those characters aren't beyond critique, either, because we still live in a world that prefers to see people heterosexually-partnered regardless of orientation, and if a bi character only ever seems to have opposite-gender relationships on screen and same-gender ones are just mentioned in passing, that's still a thing that is worth discussing (which is once again completely different from a real bi person who mostly dates people of the opposite gender, like in this example a lot of this is just about screentime which isn’t a problem for a real human being’s love life). But we're talking about characters that the fandom has interpreted as queer (often for very good reasons! I read all four of those people as queer myself) but that the creators did not intend that way and that most audiences of the time those works came out were not going to see that way. Insisting that it's actually secret subversive bi rep and if you disagree you're biphobic is particularly bonkers in those circumstances.
Remus/Tonks and Jezri were not written as groundbreaking portrayals of queer people in opposite-gender relationships. They were written to be straight, and in the case of Remus/Tonks and also I would argue Julian Bashir relative to his relationship with Garak, written to divert attention away from queer relationships and queer readings of those characters. I think it's great that fandom nowadays is reading those relationships in a way that attempts to redeem them and bring them up to date, and I'm the last person who would argue for authorial intent above all else, but I also think you need to be aware that your reading is not the intent. And that people who instead see those relationships more cynically, as attempts to suppress the queer potential of these characters, to discourage those readings, are not only "equally valid" but reacting with much more awareness of the original context these works were released in and the way that most audiences viewed them (and to some extent, still view them). And within fandom circles it's annoying enough but I've seen people say this to the original creators of works like these and that's especially maddening. Headcanons are fine, ship and let ship, but also just don't fucking give credit where credit isn't due! The people who came up with that reading for that relationship were you or other fans -- not the original creators!