He explains I believe, read on.
Are you implying that the commands listed in verse 21 and onward put a new law into effect and it is "one of the least of these" new laws that we shouldn't teach against? If so I'd like you to realize three things:
1. What he is teaching is not a new law.
They not only conform to God's Torah (not murder, etc.) but also amplify its message with such commands as not hating your brother.
2. If we taught others to break one of the least of those following commands and broke them ourselves then I believe we would be going against the spirit of Torah.
3. This passage is not contradicting any part of the Torah but is actually affirming it. In other words everything ties in well rather than standing in opposition. Verses 17, 18 and 19 show us that he didn't abolish the Torah or Prophets, not the least stroke of a pen will disappear from the
Law until heaven and earth pass away and that the commandments he's talking about that we should teach and obey not only refer to the Law as seen in verse 17 but also to its true observance within the parameters of its intent: love.
Also note that in verse 31 he was not confronting a decree of God from the Law/Torah that said divorce for any reason was okay. He was confronting people's twisting of the Torah. Christ was clearly upholding the intentions of the Torah as we see in Deuteronomy 22:29, 24:4 and ultimately 24:1. Deuteronomy 24:1 gives two stipulations that
must be met in order for divorce to be allowed: 1.) the woman has to be displeasing to the man and 2.) the only way a woman may be found to be displeasing is if he "finds something indecent about her [...]"
Other translations translate this as "he has found some uncleanness in her [...]" This uncleanness or indecency is marital unfaithfulness.
So, according to Deuteronomy 24:1, a man could only divorce his wife if two conditions were met: 1.) she was unfaithful to him and 2.) he found this to be displeasing. Note that divorce was not necessary even if she was unfaithful, but the man was fully within his rights to divorce her if she was found to be unfaithful. Culturally, the moment at which he found her to be unfaithful was after he had married her, gone away, built his house and made a living for himself and then came back to get his bride and participated in consummation of that marriage. It was at the consummation - after they had been married for a while but never together - that if he found her to be unfaithful in his absence he could divorce her then and there.
So what was happening in Jesus' time was that people had distorted the meaning of the Torah to mean that the man could divorce his wife for anything that he didn't like about her or that he called her unclean for or indecent for, etc. And this, like much of what Jesus was arguing against, was defeating the true intent/purpose/spirit of the Law with their interpretation of the letter of the Law.
Jesus was simply setting them straight. And this is what he was doing with all of the Torah. He never contradicted it or abolished it.