Okay, to be fair, there's a lot more to the story of the beginning of Arthur's reign than just getting crowned, getting married, and getting the world's largest table as a wedding gift. But it's with the establishment of the Round Table that the plot really takes off. Now, there are 150 knights who will go on to prove themselves in the remaining tales. Not only that, but Arthur has a beautiful new wife, who proves to be a source of all kinds of drama. Soon after the founding of the Table, Launcelot arrives, and the triangle of Arthur, Launcelot, and Gwenyvere will come to be very important to the events that follow.
Arthur's goal after establishing the Round Table is to have unity amongst his knights, and to command the loyalty and respect of the best knights in the world. Unfortunately, Artie, that's a bit of a pipe dream. Launcelot's love for Gwenyvere obviously conflicts with this goal, and the blood feud between the families of Lot and Pellynore splits the Round Table in two.
Launcelot's affair with Gwenyvere has been going on for a long time, but it's not until now that Aggravayne and Mordred decide to do something about it. This really throws a wrench in things, because now Arthur can't just sit back and ignore it. It forces the knights in court to take sides,(something that the feud between the families of Pellynore and Lot has already sort of done) so battle-lines form quickly. Launcelot's rescue of Gwenyvere, who's about to be burned at the stake, leaves things unresolved, though. The affair has ended, but nobody has really been punished.
All right, all right, he's been in several wars. But this one's special, we swear. He goes to war with Launcelot, of all people. Of course the only possible reason a king would go to war with his favorite knight is a woman. So this war is the inevitable end of Launcelot and Gwenyvere's affair. Arthur can't ignore what Launcelot has done; he has to punish the guy somehow. And Launcelot can't help fighting with Arthur once he invades his lands.
The war with Launcelot is still going on, but now Mordred's set himself up as King of England, while Arthur's off, fighting over his Queen. Will Arthur be able to re-claim his lands from Mordred? Will Gwenyvere allow herself to be married to England's new ruler?
Gawain's reconciliation with Launcelot symbolizes the end of the feuding between Arthur's Knights, resolving that tension. Unfortunately, though, Arthur is not able to re-claim his lands from Mordred without dying.
With Arthur dead at the hands of Mordred, Launcelot and Gwenyvere's affair is really at an end. Both of them have now devoted themselves to God. Launcelot's burial of Gwenyvere's body at Arthur's side signals his final ceding of Gwenyvere to the King in death, which is something he refused to do in life. And so ends the reign of Arthur.