In these verses, God calls you and me to a practice of marriage consistent with the holiness of marriage. What Jesus is saying here is NOT that divorce is almost always allowed because of this one exception. On the contrary, what Jesus is saying here is that divorce is almost NEVER allowed as far as God is concerned. The -exception clause- is not a broad exception, but is actually a very narrow, very rare kind of exception. Just look at the reaction of the disciples in verse 10. They understood what Jesus was saying.