This could just be me, but wouldn't you be able to use the same method used for advancing LOVETALK?
IF opposite gender, then advance LOVETALK (which would make sense, since the NPC'd risk ticking off you and your romantic interest)
IF same gender, then advance FRIENDSHIP (since, well, no romantic interest you have would honestly think twice about that)
A question on gender changing items and accidental Wild Mage surges btw:
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Edwin - "I must admit, you are quite the capable spellcaster..." (or whatever male romance talk the guy has)
*PC casts magic, it surges and the change sex effect is rolled on the Surge table*
Imoen - "...I've always loved you."
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Anyway, you get the point: Does changing genders (through cursed items or aforementioned Wild Surges) affect romance talks? Because honestly, the alternatives are rather disturbing (ending up with male PCs sleeping with Anomen (Augh! My eyes, i'm blind!) or female PCs getting Aerie pregnant).
So yeah, already brought this up in the NPC Flirt Pack topic, but it's been bothering me how they're planning to tackle that one in-game. Technically, when the PC's gender is set to male, the LOVETALK for female NPCs should be able to go up. Yet that would still leave the matter of 'loose ends' with the active (in this case male) relationship, as well as the newly started female relationship when the effect wears off, or the item uncursed. If it doesn't, you'd get gay relationship options (note that i am not against gays: it just leads to unintended ingame situations)
Either way, it could serioursly mess up your ingame romances, even disregarding the mental imagery i've just thrown at you in this post...