I've been wondering the same thing. There seems like there are a lot of indications that you should be able to, but I can't find a way to actually make it happen.
Technically you can't fuck your slaves, but you can fuck your servants, so if you want to get down and dirty with a gal you got to first raise their love stat above 0 then you can assign them to a room in the Lower Servant's quarters.
I have servants in the lower servants quarters at 100 love and there still doesn't seem to be an option to have sex with them. At least not one that I can find.
