Player-sexuality is not to be confused with "Everyone is bi"; that's not what it is. It's a statement that IF a player finds that their personal character is attracted to and wants to pursue a particular NPC that they have grown to like and would like to feel a rapport with, then that desire will be able to be met, and the game will define itself to accommodate that.
Yes; characters in games like kingmaker have that slightly more intricate definition, because those details are defined and become a part of who they are... however, it also causes problems; suppose you are a male-preferring male, in Kingmaker, for example.... What are your options? Who can you pursue attraction to? One character, and one character *Only*, and *only* if you're okay with breaking him up with his current partner first.... Is that satisfying? No, it's not. It's not feasible to cater to all player preferences as well as character type attractions, with hard-defined characters: you'd need a homosexual male academic scholar type, a homosexual female academic scholar type, a heterosexual male academic scholar type, a heterosexual female academic scholar type, a homosexual male roguish fortune-player... etc., etc.,... it's not feasible.
Making romanaceable characters player-sexual doesn't mean they don't *Have* that same definition that can be a part of who they are - it just means that it's not determined until *you* determine what it needs to be - it is and becomes a tangible truth, as needed. Those exact same characters can still have a 'default' written preference to follow if they aren't pursued by the player, but the player's needs, for the characters they feel that they want to pursue, need to override that. That may sound crass to some, but we have to remember that it's a fictional story being built with fictional characters; no-one's personal will or freedom is being taken away or denied by doing this.
Ultimately, it's about asking who the game is for: is it for the players, or is it for the world-authors? As a DM, your game world should be for your players - not the other way around. In tabletop, even if you hard define a character's preferences as part of your world, it's softer, since you can still supply alternatives and other options for your players... in a video game, that isn't as feasible at all; we have a limited selection of options, and that's all we have. They *Must* be able to be what players need them to be, because they are our only options. That means that, in a social romance setting, players need to be able to create the character they want, and then, to pursue the *type* of character personality they want to pursue, and know that they stand a chance of being reciprocated; anything less is shutting players down to a greater extent and loss of satisfaction than you could ever hope to gain by hard defining those specific details of the character. This doesn't mean that those NPCs can't have individual quirks or preferences alongside this - they most certainly can, and it can make for cute or amusing discussions! But overall, the player has to come first, and they need to know that, overarching whatever individual quirks or eccentricities an NPC has, inside the bedroom or out of it, that their efforts stand a chance of being reciprocated when they direct them towards the character they want to share that relationship with.
If a character's personal sexuality makes up such a large part of their personality and character depiction that it cannot change on different play-throughs as needed, then you have a bigger problem by far already. It shouldn't have much of an impact on the presentation of the character and development of their personality - it can afford to be what each individual player feels they want it to be, without detracting from the character, the world, or each person's individual experience of it.
Yes, all of this. Well-said, Niara. As you know, I don't always agree with your every opinion, but I do always appreciate how much thought you put into them, and how articulately you express them. In this case, though, I do definitely agree with you.
(Well, sliiiiiight issue with "As a DM, your game world should be for your players - not the other way around" but not gonna get into that. The core of your point there is still good.)
A video game, certainly, is for the players. Giving them more options is always good. Avoiding scenarios where someone feels left out of what they want is always good. I will always remember how sad it made one of my gay male friends that he couldn't romance Alistair in DA:O. It ruined the game for him. Some people may find that silly, but his feelings were valid, and not unique to him.