I'm at Loncon 3 right now, mostly attending talks of professional interest, plus the odd, film, play, etc. Here, I'm going to talk about the first two panels I attended together because they had many points in common.
The first was about the representation of non-traditional families in SF/fantasy, the second about how we represent non-human sexualities, for example vulcans or vampires (but not gods, despite the long history of that particular trope). In a way, the first talk had a wider field to choose from. There was a focus on gay relationships and polyamoury but more traditional alternative families were mentioned: single parent, recombined, adopted... some of these alternative families are in lack of representation at the moment, others have been tropes since time immemorial. Some of this may be due to social prejudice but because of the way the talks developed, I think it's worth considering the narrative reasons a bit harder.
It's a trope to start a story with a family that's just been split apart by death (or divorce in the modern world), partly because it allows the author to begin at a moment when the characters enter a new situation (or at least radically changed conditions), are already under stress and, lets face it, in a position to elicit reader's sympathy. It's a trope to start a story with an orphan in search of family, for the same reasons and also because it allows the author to cut the backstory to a minimum. Here, more than anywhere else, the protagonist starts with a cleanish slate and a usually a desperate need.
At the talks I heard today, people were keen to read and write about non-traditional families and sexualities but they still saw it as a challenge to be overcome. They mentioned the usual concerns about acceptability to a wide readership and commercial pressures from publishers and editors and a whole lot of other issues. But there were some concerns to do with the craft of writing which transcended both talks.
It's in the nature of SF/Fantasy that the story is often about a quest, a political upheaval, or other adventure. This is what these authors/readers wanted, but with characters that 'just happened to be' polyamorous werewolves (or whatever). Their narrative problem was that as soon as they strayed too far from the common ground of so-called neutrality, they needed to fill their readers in on the details of their protagonist's situation. If they weren't actually entering info-dump land, they were at least introducing undesirable amounts of distraction from the main plotline.
To minimise the narrative impact of the character's alternative lifestyle and also because they desire this for our future, many authors let ther characters live in worlds in which their lifestyles attracted no particular attention. The alternative: dealing with all the conflicts, the juggling of priorities, the closeting and passing and asserting of rights would distract from their desired storyline even more.On the other hand, so would dealing with the structures and institutions a supportive society might be expected to build around accepted lifestyles.
I think writers would be probably be best served by addressing the issue of the lifestyle they want to introduce head on as an aspect of world-building, plotting and character design. It is actually bad writing to have a character who 'just happens to be..' It isn't done with 'neutral' characters either - they just require less leg work. The character's situation, their society, and the people in their lives need to be built into the main storyline, as antagonists or allies, obstacles, opportunities, motivations, providers of stakes and sanctions...
I introduced the idea of cognitive overload into the title of this post, as something which describes the way this overload of 'stuff that must be dealt with' applies to real as well as fictional people. It's hard to write someone living a minority lifestyle in a not altogether supportive society while getting everyone to focus primarily on their pursuit of some other goal. In real life, the same burden falls on real people and gets in the way of their goals too. It's a real social problem and if we can't overcome it even in fiction then we should be doing a lot more to acknowledge its impact on the lives of real people.
The talks I attended were: Reimagining Families with Jed Hartman, David Levine, Rosanne Rabinowitz, Laura Lam, Cherry Potts and The Domestication of Spock with Jude Roberts, Nick Hubble, Mary Anne Mohanraj and Justina Roberts.