Arthur and Gwen both had a huge crush on Merlin but none of them know if he likes them back, and both of them are convinced that Merlin likes the other.
Arthur: okay, I have an idea
Arthur: we're going to start to date each other. And when Merlin gets jealous - we have better chances at finding out of whom he's jealous.
Gwen: This is such a bad idea. I can't believe I'm desperate enough to try it.
So, they start dating each other. Arthur tips the law in some directions so it would be possible for a noble to marry a servant. Which is definitely not a ploy to make sure Merlin knows he's willing to alter the entire kingdom for love-
But then Merlin tells them both how good they look together and he seems a little sad and conflicted, even when he's happy for them. Gwen and Arthur, at this point, don't think they can turn back anymore. Especially when Merlin says to Gwaine one time, drunk, at the tavern: "I could never date any of my friends' exes. That would make everything awkward. Which is really a bummer. Because I think I have feelings for both of them! Not that they'll ever break up. Not for me, anyway." (Gwaine is in on the ruse, so he tells them. Arthur and Gwen nearly faint.)
Gwen: oh god, Arthur. I think we have to get married. If we break up, Merlin will have to give up on one of us and the one he remains friends with, will never get to date him.
Arthur: shit, I think you're right. If it's any consolation- If it had to be anyone besides Merlin, I'm glad it's you.
*awkward moment where they smile doply at each other* *both realizing*
Gwen: do we like each other?
Arthur: oh thank god you said it, I would have repressed this for years. I think we do.
Gwen: and our feelings for Merlin?
Gwen: ... You know what? I have a crazy idea. What if we invite him to join us? We could be, what's the word- poly?
Arthur: ... Gods, I think I'm actually in love with you
Gwen: ... *Blushes anyway*