They’re breaking up.
My first playthrough was a mage where they broke up, and he chose to be King. My mage chose against the ritual and had told Riordan they'd make the killing blow on the Archdemon... turns out doing that and having Alistair with you in the final fight causes him to stop you and tell how much he still loves you... with a kiss he'll charge off to kill the Archdemon himself... I don't know what's worse, the break up or that.
It's both.
And yet, for me, it's canon. I never said my character was okay after this, but her story does continue in Awakening.