In the most recent community update it says the following:

To offer further clarity on exactly who can romance who in Baldur’s Gate 3, we’ve laid it out in the least romantic way possible, below:

- A played Origin can romance an NPC Origin
- An NPC Origin cannot romance an NPC Origin
- Played Origins cannot romance other player Origins

Seems pretty clear that (a) only the Origin companions can be romanced, and (b) only by another played Origin. This would imply that custom PCs cannot engage in romances. Is this correct?