His (the parson you want to marry) parents do not have the right to deny your right to marry. Cause, his parents don't have that right, your parents have it. If your parents permit you to marry him then, he may marry you without the permission of his parents.
If a man's guardian deny him the right of marriage then, he may marry himself without the permission of his parents. However, the permission of the woman's guardian is required.
P.S. You should remember that he is already married. He may marry you as a second wife but he must not do injustice to his first wife.