It is commonly and academically said that the episcopal ring signifies the spiritual marriage of a bishop with his diocese. But bishops change dioceses today like men change their consorts, alas!, shuffled around like so many pieces on a chessboard, even often themselves vying for another post, all under the authority of the Roman Pontiff. So the ring surely cannot mean a permanent commitment to the diocese the way the wedding band signifies the vow of "until death do us part." There is no such vow on the part of the bishop to wed his diocese. Would that it were so! It would solve some major power issues of the Church today. That would also, incidentally, eliminate the retirement of the bishop.
Bishops move and leave their dioceses for a further appointment by the Pope, and bishops are required by law to retire at age 75. In both ways the bishop's commitment and relationship to his diocese is not like marriage, not at all permanent. The bishop's commitment to the diocese is a temporary contract, let's not confuse holy, indissoluble, marriage with that ephemeral bond.
We certainly need a better interpretation for the bishop ring.
Cf. "The ring is the symbol of the spiritual marriage of the Bishop with his Church. The pontifical ring, adorned with a large gem, must be loose enough to be worn over the gloved finger." A Synthetic Manual of Liturgy, Arian Vigourel, Baltimore: Murphy, 57.