It's simple, for Npc's with those preferences the roll chances should be automatically failure
Okay, so let's flesh this out and make it a constructive suggestion, if you're serious about it...
To work the way you're suggesting, each individual NPC (or at minimum, those for whom persuasion options are to be written) would have to be written with an existing intimate preference in mind. There are a couple of ways you could pitch this, and I'll leave it to you/ask you to suggest which way you'd like to pitch your idea:
Option One (Flexible NPCs): In the realms, and as gently guided by developer comments and the DMG in more subtle ways, it's common practice for the vast majority of NPCs to flexible - bisexual or undefined until a situation or detail requires an inclination. If you you were to follow this trend, then there would be no automatic failures, because the NPCs so-propositioned would always be at minimum open to the offer at a theoretical level.
Option Two (Fixed NPCS): As much as the above is common advice and world-building tool-kit, the reality is that most people, in their home games, run NPCs with personality and detail that will often as not include an intimate preference, and the soft universal bisexuality of the realms is overridden with characters more narrow an diverse. There's nothing wrong with this either; in game, this would require that every NPC that was to have such flirtation options would need to have this detail of their personality defined, however, and along with lines and responses, that would be a lot of additional work. In this structure, you'd also want fair representation to be present across the spectrum; you'd equally want to have bisexual NPCs, heterosexual NPCs, and both male and female homosexual NPCs as well. Short of other dialogue or earlier information and clues, the player is not going to know in advance, and the opportunity for disastrous
faux pas is, I admit, amusing.
An issue with the second option is that Larian have already tipped their hand in regard to the direction they're taking these matters - All NPCs seem to be at the very least player-sexual, we can choose to separate our sex from our gender identity, and when we do so the entire universe and everyone in it psychically knows about this in advance of meeting us, and respects it completely no matter how puppy-murderingly evil or insultingly disdainful of us they otherwise are. As bad for immersion as that is, that's what we're working with, currently.
Beyond this, regardless of how you choose to set the system up, it needs to be accessible to all players - so we'd need to have both male and female propositions lined up for each instance where such a proposition was planned - and generally speaking the way women flirt with women, women flirt with men, men flirt with men, and men flirt with women are all subtly (sometimes not so subtly) different; The usual caveat needs to be stated here: everyone is different and there are no universal truths about this, just trends... but I can certainly say that the way in which I flirt with or make an offer to another woman is substantially different from how I approach doing so with a male I'm interested in. So how would you suggest this be handled?
We can try to make the offers tailored to each instance and each NPC that they're intended for; given the game's current structure you'd need at least three distinct options to be coded and developed for each NPC that was persuadable/able to be propositioned in this way, and unless they're very few, that would be a monumental amount of work. Alternatively, if we don't make each one custom, then we'd need a generic instance of the proposition that could be fit to every persuadable NPC instead, and we'd still need several of them to account for different configurations - less work by a long shot, but also cruder and less satisfying at a story and immersion level.
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Lastly, it's best to understand that from Larian's perspective, the game is now feature complete (or nearly so),and at the polish stages of development. They aren't going to be sourcing new features to implement from these forums or any other social platform at this point. So, it's worth bearing in mind that what we discuss here is ultimately going to do its best work inspiring a mod-maker to develop something to suite this idea, come release time.
With that in mind though - we can certainly flesh out the idea into a proposal that someone could be inspired by.
