To be clear, the companions are player-sexual, not bi-sexual. Each companion seems to have some sexual preferences about other NPCs, revealed through their history or dialogue or banter, but always wants to bang the PC.

If companions remain player-sexual, I think their sexual preferences should be at least mentioned. E.g., Wyll and Gale (mostly presented as straight) should, when flirting with a male PC, have some dialogue line similar to "I've never felt this way about a man before." Whereas Astarion (who flirts with all companions during banter) wouldn't have such a line. Companions should have a defined personality and desires that remain consistent within the game, and any deviation from that should be adequately explained or else it is jarring/unbelievable.

Alternatively, I'm fine with companions having set sexualities like in DAI. However, this requires there be a sufficient amount of companions. CP2077's companions massively failed because there was only 1 option for each gender-sexuality pair. BG3 would need a minimum of 8 companions. For each gender of companion, there'd be
1 straight
1 gay
2 bi
This would ensure that every PC has 6 available sex romance options--3 of each gender--which seems sufficient imo.