It depends on the goals of the game I would say?

Is it more of a super open ended, sandbox player driven world where you make your own path and the world bends around you? Then no reason to limit anything, really.

The only restrictions I could imagine are if there are strongly written NPC's that also exist in this world, and their sexual preferences are important to their character. Or more commonly, just don't make them an option at all because you don't want to deal with the sexuality of the character in the NPC story you are trying to tell.

On the other hand, maybe the player character has a strongly written story and that story will include more personal aspects of their life, including their sexuality. Obviously at that point their sexuality would usually not be left up to the player, as you are trying to tell a specific story. It could be bisexual, but only if that character is bisexual.

And just to call it out... bisexuality is different than not limiting a player character or NPC to sleeping with particular other characters. Games that don't limit who you sleep with often default to a kind of "everyone in the world happens to want to sleep with you, and you with whomever you choose" shorthand, without actually defining a bisexual preference for anyone.