David was not a good Christian then, no, if he was, God wouldn't of rebuked him so strongly. He paid a hefty price for his sins. Difference between David and fornicators of today is, David repented, and went through a season of godly sorrow. This is evident in the Psalms. Now people are told its OK dont worry about it, just confess and keep on keeping on. Nobody is perfect, right?
If you have a wife, by definition its not fornication. Do you know what fornication means? To have sexual relations with your wife, is not fornication. David had many wives, as did many people in that time, there is nothing in the Torah that forbids a man from having multiple wives. God never confronted anyone in the Torah about that, however today, we have the teachings of Jesus in full-effect and as Jesus said, God meant for us to have only one wife.
This is why the Paul teaches that a leader in the church should be a man of one wife, because there were most likely many jews in the early church who had converted and already had multiple wives.
But all this is not related to the topic at hand. Truth still stands, fornication is not lawful to a Christian, and the verse you quoted to say all things are lawful should be read in context. The same Paul condemns all such behavior in the chapter just before that.
I have said what I got to say to you, I hope and pray God opens your eyes and that your conscience isnt seared yet from sitting under this kind of teaching.