Reported by JDR: if you use voodoo doll to enter the council and later reenter it posing as messenger, you don't get the sex option from librarian for whatever reason:

Originally Posted by JDR13
I figured it out..

Not sure if it qualifies as a bug, but I can confirm that you have to enter the high council building as a fake messenger to have that dialogue option. If you choose to just use voodoo and not help Azuro, you will not have that option with Cassandra.

Thanks JDR!

After a few days I'll edit the first post with all things reported here and also repost it on official Risen2 forum hoping Deep Silver support will notice it.