If we select a female character (the woman of our dreams), we should get (and be able to offer) romance invitations with females.
If we select a male character (the man of our dreams), we should get (and be able to offer) romance invitations with males.
What if I want propositions from both?
Well, good point, and that is why I also wrote, that Astarion, and maybe another female character could be neutral.
I also think it is not really logical to have propositions from the whole group anyway, so 2-3 max should be enough.
(Yes, yes, and what if I want a "party" with 5...
Well that option is for whatever reason missing form the game.)