I would probably be more fine with all the companions being playersexual if they weren't all playerhorny. And, at least as of a patch or 2 ago, incredibly jealous and snaky if I slept with another companion.
Originally Posted by Niara
I will argue strongly for the use of playersexuality in games with romance, for reasons I've stated; most importantly that a character's persona and characterisation should not revolve around their sexuality, and that making it a defining feature of a character is a failure in writing... but I will definitely concede that at the moment, Gale and Wyll do feel a little bit jarring in this respect with male PCs
[snip]
One important thing I feel makes playersexuality work well (though this is harder to manage) is that the characters CAN have predefined leanings that they default to without the player, but that they shouldn't be dominant enough to be defining traits, and that they should only show up when the player isn't pursuing their romance. This helps divorce the concept of playersexuality from general bi- or pansexuality.
Strongly agree with you that, if playersexuality is implemented, then companions should still have predefined leanings. Every additional opinion or preference an NPC has makes them more well-defined characters. And, as you suggest in the snipped portion of your post, having dialogue that reveals those preferences and then explains why they're attracted to the PC and how that relates to those preferences is great.

Without that additional dialogue/explanation, however, I think I'd prefer that some NPCs only be romaceable by certain genders. Like Gale: his history and party banter strongly suggest straight so it breaks a bit of immersion/his characterization that he's so willing to sleep with a male PC.

I'll qualify the above with: there needs to be enough companions to not make this overly restrictive. If BG3 eventually has 8 companions, then there could be 4 who have a set sexuality (the combinations of male/female and straight/gay), 2 that have stated preferences but can be woo'd regardless of PC gender, and then 2 that are openly bi- or pansexual. This would allow anyone the option of 6 romanceable PCs (3 if you only romance companions of a single gender).

tl;dr: Basically what @kanisatha said, but I'm fine with all companions being playersexual as long as any conflicts between their banter/history and their attraction to the PC is explained.