Marriage to Divorce

I come from a large, crazy family.  I have two brothers and four sisters.  My parents have been married forever.  My grandparents were married for 63-almost 64 years when my dear grandfather passed away.  The trophy hubby and I will be celebrating our 20th anniversary in a couple of months.

Yet while we have a nice history of long, relatively happy marriages (seriously- there is always something new to deal with!), there are plenty of divorces and broken families.  Mental illness, addiction and simple selfishness have all played rolls in the decline of various relationships.

Being a Christian, I'm told that marriage is a union designed by God.  Marriage should be considered a sacred union.

Yet I've seen some relationships that are truly toxic.  One partner tries to be a good spouse while the other is an addict or suffering from a mental illness that they refuse to get help for.  And don't even get me started about when kids are involved.  Separating seems to be the best solution for all involved.

So where is the line?  What do you consider justifiable- religious or not- for terminating a marriage?