Is it difficult to tie his belt; and furthermore let him get up and pray as he is; and the גמרא answered because it is written, ישראל prepare yourself towards your G-d - טריחותא למיסר המייניה ועוד ליקו הכי וליצלי; ומשני משום שנאמר הכון לקראת אלהיך ישראל
Summary:
One must gird his loins when he davens. However if one wears a belt on his pants he is not required to gird his loins (to prevent לבו רואה את הערוה, according to the מחזור ויטרי).