Too many young couples get married in the Church, not out of faith, but out of pressure from their parents. After they’re married, they rarely if ever attend Mass.
The point the former Pope is making is that this could invalidate a marriage and needs to be taken into consideration by marriage tribunals in the annulment review process.
So, back to Jesus words, those asking the question were trying to trap him, so he gave an answer within this context. He was also speaking of a valid marriage, and what that meant to Jesus he explains in that it is what God has joined. Lack of faith at the time of marriage leaves God out of the act the couple makes and thereby makes it an invalid sacramental marriage.