The case Jesus was describing was divorcing your spouse to marry someone else. That would be adultery. It does not describe previously failed marriages or cases of violence or alcoholism. He also emphasizes that when someone is married, they are no longer members of their family of origin (indicating that maybe the divorce in question was about social status and family rlations and not just lust). This last statement, shows why this passage is not a condemnation of gay marriage. Rather, it endorses the fact that the family of origin is now in second position behind the spouse, straight or gay. While what Francis is doing is a good first step, recognizing that divorce is necessary sometimes as a divorce rather than an invalid marriage is an adult thing to do, as is recognizing the rights of gays and lesbians.