The Law was given by Moses, right from God's mouth on Sinai. Nobody ever suggested Moses departed from the very Law he received from God. Rather, Jesus appeals to Genesis, before the Law was given to undermine and  make ineffectual the accepted Deuteronomic laws. Your second question is a good one and I haven't any answer. I will have to think about that. You may have intuited that I do see room for divorce and I do, simply because I have know so many divorced people who shine with the light of Christ. When you know concrete circumstances, abuse alcohol, etc., how could anyone accuse them of "serial adultery."