Jesus teaches two very clear lessons in Mark 10:2-12. Man and Woman at consent become one flesh and human beings cannot separate what God has joined. Jesus also teaches that if a man divorces his wife and marries another he has committed adultery. In this video the follow are discussed:

Who determines the validity of Sacraments (including marriage)?
Why are Catholics bound to the Sacraments?
What is fornication, cohabitation and adultery a sin?
What can someone do if they are in sexual sin?
What exactly is an annulment?