SureAI

Is the PC's orientation set in stone after the conversation with Jespar in the tavern? I wonder because, as the intended player of a bisexual PC, I noticed there's no option that really lends itself toward seeming such -- you can choose to comment on the men, the women, or... the food, apparently, which seems like a very disinterested thing to say. I assume that one means 'friendship' or 'platonic' storylines for all potential love interests?

I suppose it really only came up because Calia seems like a wonderful person with a wonderful story, and I worry that a dialogue line I said earlier to Jespar may have locked me out of exploring Calia's subplot!

I don't think it's set in stone just because of the one thing you said to Jespar that early in the game. My theory is that it is decided which partner you get at the end much later in a scene where you get asked a specific question. Don't read if you don't want to get spoiled^^

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You get told to go to bed and rest (you can do things before that, of course, but then, off to bed with you^^), another scene is playing and then you wake up in bed, in my case it was Jespar who talked to me/woke me up. A bit of talking followed and then,as if it was an afterthought, he asked me about my feelings concerning Calia. IMO that's when it get's decided which romance path you follow.

Concerning the subplot with Calia, you can follow through with all the quests, except one, I think, which is the last one. But that's again, spoiler^^ and since I don't know anything about the very last quest with her, someone else has to comment on that.

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But in the end, it's going to be some romantic talking to each other or something, ending with you both going to bed together

I complimented Jasper on a few occasions without any love interest and he started hitting on me. The game gives you options to stop these advances but it seems to remain a possibility until the very end if you keep a friendly relationship. As has been said already the game decides your real partner much later in the game. If you've been friendly with both of them then you will get asked by one specifically what your feelings are about the other. This scene should be the final decision (the one Cyrenne already mentioned where you awake from the bed and get approached - although another such questioning incident occured at an earlier point already with Calia). You should not miss out on any character side quests before that point.

Wonderful, thank you!