"Diocesan bishops are at all times subject to and bound by the Church's Constitution, Canons and Book of Common Prayer. None of these documents authorizes a diocesan Bishop to waive, to declare null and void, or modify or amend any of the Church's Constitution and Canons."
And here is the Episcopal Church (USA), as its diocesan bishops act in real life:
While we remain in a provisional time when our canons have not fully caught up to what I believe is an intersection of the movement of the spirit and the understanding of the people, it seems that now is the time to remove any distinction between same-sex marriage and other marriages. From this date forward, please simply follow the canonical requirements for marriage regardless of the gender of the couple.