When someone says that "Greer has always stood behind the sanctity of marriage", that means he refuses to allow a divorce action.
Theodore is being sarcastic about the words, "sanctity of marriage", because the hypocrite Greer does not seem to care that Michael is living with another woman and fathering two children by her.
This fact should legally disqualify Michael from being Terri's Guardian, but Greer refuses to hear that motion in court, thereby allowing Michael to get away with defiling the sanctity of marriage while ordering his wife to be abused/neglected and ultimately starved/dehydrated to death.
Thanks, honestly I didnt get the sarcasm in the original post so I appreciate a dummed down explination.