>>and don't tell me that this kind of stuff is for adult, i'am not advocating here details descripton of pratices like there is no detail practice of drugs in the game. do you really think that a game that clearly show tools of torture is for childs?<<
I don't think the game avoids discussing sex, sexuality or prostitution, I imagine it is something which simply doesn't come up ... just as it doesn't bother discussing every type of media available; the most common sports/arts practiced in the empire; human racial boundaries above and beyond common planetary social structures; naming conventions for children; etc. Sexuality and prostitution (along with homelessness, begging and pan handling as you mention later) are merely another part of life left for you as the GM to fill in.
You are correct, prostitutes can be a valuable source of information - I've had players choose to be prostitutes in games (yes, female players) - and in a recent game I had a player build a web of informants out of the homeless, drug addled scum of the streets. (though the problem with the homeless and drug addled is: if they'll talk to you for a meal and a fix, they'll talk to anyone else as well)
If you would like to see an extensive discussion on these topics, start a thread in the forum, I'm sure you'll get some responses.