As to actual defence of the system: I really dislike it when all companions want to sleep with PC at once - it's just a barrier from making them feel like individuals, rather then "pick your sex cutscene". Therefore, if every/most companions are romancable, I believe there needs to be some system that limits what companions are available for romance in each playthrough, based on the character you play or your actions. Different games have different system - in PoE2 for example, companions seemed to respond to their reputations with you but also [I think] to your dispositions (aka honest, benevolent, cruel, etc). Though that system didn't quite work as intended in more ways then one... At the very least in 2.0 it spread companions hitting at PC around - so the best fitting choices would initiate romances early on, while others in mid-late game.
So far BG3 seems to be doing a good job - in my playthroughs I seem to get friendly with one or two companions depending on my choices. And so far it makes sense.
The only I could do without, is companions letting me know during the party that they would totally sleep with me, if I did more things they like. That just feels weird.
Honestly it's exactly this scenario that makes me think that adding another layer of depth to the romance mechanics is necessary. All of the companions come on very strong during the party scene- which sure, as a romance-minded player, is very funny to me, but ultimately is a little tiring.
That said, I think that you and others bring up good points about how the specific layer I suggested has its own flaws. If you don't mind me asking, where do you feel PoE2's system failed? I think that a game like BG3 has an opportunity to make party romance just as multifaceted and interesting as its other functions, so I'd like to hear opinions about other games' attempts at similar mechanics as much as possible.
(Of course, don't feel obligated to respond.)