Well... I thought long about this suggestion... and especalliy about the "it´s realistic, because it existed in the medieval time" and yes you are right, but there is another side of the coin: A women who prostituted herself in this time, was an outcast, shunned by the church and the society (I know it was bigot, because everybody used their services) and normaly she only did it because she had no other choice. But what would be the next step? Allow rape, because it was also one of the things that existed, to brake the moral of the enemy and boost the moral of your own troops? Because its realistic? Carefull which terrain you walk.
I understand why some want it and why some don´t want it.
If there should be Sex introduced in the game... where are the children? Should the Alchemist be able to abort an pregnancy or brew contrareceptives? What about the high child mortality? What about the STIs? Do we need goodwifes? Because it would be realistic. Yes, it would add a lot of gameplay, but usefull gameplay?
On the other side, yes it would a new scale of gameplay to the game, but it should be something really voluntary. You could get engaged with a ring, marry, and so on, good roleplay elements. There could be proposals between Kingdoms. And if you really like, you could have Sex (in your room, with an animation under the blanket).
Two extremes... and I am frank, I don´t know a perfect solution. Without any "sexual elements", the game would feel a little bit hollow, but its part of a society, but if the main theme of the game becomes "how to get somebody to fuck", then it gets annoying. So there have to be something in between.
My opinion about the technical thing: Why should we need an extra building for that? Use a building that exits, and build themebased funiture (hopefully we get MORE furniture in the future) in the rooms. And hopefully we can add signs to our houses some day. ;-)
As allways sorry for my english, I am not a native speaker.