Your comment implies that Jesus is contradicting Himself, but I think the true meaning is based on how we are to understand the phrase “except for porneia”.
Historically, and in context, the church has understood this to mean, “except in the case of an unlawful sexual union”-— that is, if the first union were not a valid, God-made bond to begin with.
What’s an unlawful sexual union? Does porneia include incest? Seems like it would and that was necessary, at one point when Adam and Eve’s offspring had no other options.