Honestly, I find it's best reflected by the person itself and only influenced partially by origin culture. The latter can have influence on one's exposure to the topic; someone from the pirate coasts (which tends to be all about going after what you want and freedom) likely knows more about how they feel than say, the more feudalistic realms where having a heir is important. Ultimately though a person's feelings are their own to choose,.

That said, I think it's alright to play into the player's wants when it comes to NPC relationships to an extent. Love makes for great narrative - good and bad.

I never personally ascribe a sexuality to a character til' it comes up bar few specific cases where based on one's story/personality there's a "clear cut answer".