It's important to remember, all of the origins are playersexual: how they act in their companion romance is not reflective of the canon the player creates for them when playing as them. It is not a clear conclusion that any of the origins are monogamous: because any time the player plays them, they can pursue the poly arrangement.

Gale happily romances Halsin and Astarion, or Halsin and Shadowheart. Gale loves kissing his two lovers.

Same with Wyll, same with Karlach, same with Lae'zel. These are characters who canonically act poly in poly playthroughs of their origins.

Playersexual means both the romance options are always hot for the player character, and it means that the player decides the sexuality of the character under their control, monogamous or polyamorous.

The way origins express their preferences while not the player character is just as valid as when they are the origin, but we can't say that any of the origin characters have a hard sexuality the way the recruitables we don't RP, like Halsin/Minthara do, because when we play them we make them how we like for that run.

It's nice to have a game where all the playable characters are available for being poly and pursuing non-monogamy. Like, they could have coded it with preferences: if you play as Karlach, she only will date Shadowheart or Lae'zel if she's lesbian or something, or if you play as Wyll, you can't have the option to romance both Shadowheart and Halsin if Wyll is monogamous. But they didn't do it that way. They are playersexual characters.