Viewers control what gets censored more than any editor, executive, or FCC censor. First of all, for a network to get fined, viewers gave to call in a complaint. The FCC doesn't act otherwise. A network can show all kinds of nasty stuff all day long, but if no one objects, the FCC doesn't care. Obviously this is why the raunchier stuff was reserved for late night: fewer viewers, so less of a chance of a valid complaint. Basic cable is a bit similar, in that if a cable provider gets too many complaints about raunchy stuff on a basic cable channel, it can get dropped, but at least they do not have to worry about the FCC.
One more thread these broadcasters have to tread is not to offend advertisers. This is more of a self policing policy, of course. Advertisers have a different set of criteria as to what is offensive. A network dependent on company X's advertising dollars can not put a show on that is contrary to the interest of company X. There is endless examples of this. This is a much more insidious form of censorship, in my opinion.
In the end, what is acceptable is mostly determined by the viewing public. Anyone who thinks otherwise is not following the money. Remember that for network TV, the end product isn't some TV show or advertisement; it is you, the viewer. You are delivered to advertisers. They are the client, and you are the product. This relationship is changing very slightly thanks the profitability of TV shows released on DVD, but it is still largely intact.