A reasonable set up episode. But they don't seem to have much consideration for how Arthur's dumb decisions would actually be viewed by his people. As you say, aside from her being attractive, possessing pale boobs and not visiting the kingdom purely to murder him with magic, marrying her would also have made everyone safer.
Also, it's annoying how yet again, Gwen being the hottest woman in the land is a crucial part of the story. I mean, I can believe in dragons (when the writers remember dragons... it's been weeks!) and I can believe in magic, but Gwen is not a supermodel. And it's made more apparent by Morgana popping up on screen every few minutes looking ridiculously hot.
Oh well. At least Merlin finally told Arthur about Uncle Aggravating, even if the editing ruined it by having Arthur angry and disbelieving at Merlin in one scene and threatening to exile him, then in the very next scene asking him for important advice about the future of the kingdom.