Basically, the Wife of Bath gives a number of religious reasons for not being chaste. She does say that being chaste is best, but she argues that it is not for everyone and that God does not expect everyone to be chaste.
The Wife of Bath says that she really likes to have sex. She says that this is why she likes to be married. But she does not think this is shameful. She thinks that God has called different people in different ways -- he has made them all different. He has made her as a person who is able to have sex and likes to do so. Therefore, she reasons, there is no reason for her to be chaste.
Her reasoning, then, is that God made most people to be sexual beings. Perhaps perfect people can be chaste, but most people are naturally going to be sexual and there is not really anything wrong with that.