This is indeed a bit of weird logic on Pope Francis' part! The marriage is 'invalid' because a couple has the option to divorce down the road? If we follow this through to its logical conclusion marriages could never be valid unless divorce was illegal thereby guaranteeing permanence. That's just ludicrous. The fact is that 99% of couple who stand before the altar to marry intend to remain with their spouse. Events down the road may affect this commitment, but that doesn't reflect upon the intent of the couple - and it's their intent at the time of the marriage that is required to establish the validity of their sacramental union.
I like Pope Francis... but I think he's off the mark with this one.