I think it was one factor. A big one. He was also severly wounded at Saratoga and that wound took him out of active duty and got him appointed as military governor of PA. That was a mistake...he met and fell in love with a beautiful young tory and got a severe reprimand from Washington for how he conducted his office...even though Washington felt sympathetic to Arnold’s side. Taken all together you can see how Arnold would fall.
Chinese proverb: A journey of 1000 li begins with one step.
Arnold’s entire career might have been changed had Gates not hogged the limelight over Saratoga.
But I agree there could have been other contributing factors—promises of preference or money, a perception that the war was lost, etc.
I’m not persuaded that his love life was a primary cause since he couldn’t tell her of his change of allegiance without jeopardizing his life and of course there was the question about how she would react to his dishonorable acts in a era when a man’s honor and reputation were all important.