Haywire
He doesn't know why he says it. All the time whenever he is near Alec's mouth just goes haywire. He knows that what he is saying is untrue but that doesn't make him stop. Saying that he loves Jace all the time is the biggest crime he could ever commit but still he isn't punished, on the outside.
Inside him the punishment is commencing. His punishment is not being able to love Magnus. He's tried to. He's been forced to. He wants to, but still his head and mouth say he loves Jace.
All the time he wills his heart to take over his body. Anywhere and everywhere he wants to scream his love for Magnus. His heart and soul are to weak. No one shall ever know of his irrevocable love for the spiky haired warlock.
Except there is one person who knows. If your thinking Alec told them well your wrong. This person thinks nothing of it but it's still in their mind every second of everyday. They just don't notice. How they don't notice I have no idea but not once have they ever come across this fragile piece of information. Their instincts are too strong. They won't let them self know of the information. It will destroy them.
After all … to love is to destroy.