When the Church allows for marriage annulment is this an exception to Christ’s edict: “What God has joined together, let no man put asunder”? What is the rationale for this exception?
It is not an exception.
If I said to you: no person may vote twice. After you finish filling out your ballot you put your ballot into the computer scanner it spits it out and didn’t count the vote. Would it be an “exception” to tell you to resubmit your vote? Would resubmitting your ballot mean you voted twice? Of course not, you didn’t vote yet because your first attempt wasn’t valid.
An annulment says that no valid marriage ever existed, therefore there is nothing to “pull asunder.”