WORCESTER — Attorney General Martha Coakley is supporting a gay couple's legal battle against the Diocese of Worcester after the pair was allegedly denied the right to buy a church-owned mansion in Northbridge. Two Sutton men, James E. Fairbanks and Alain J. Beret, a married gay couple, filed a civil suit in Worcester Superior Court in 2012 against the diocese and its real estate agent after their offer to buy the Oakhurst Conference and Retreat Center, a 44-bedroom mansion in the Whitinsville section of Northbridge, was rejected by the Diocese of Worcester.