When Defoe uses the term "well bred" he really means a woman who is cultivated and cultured.  "Well bred" has to do with education, not with parentage.  He argues that a well bred woman is simply a more pleasant person than one who is not.  He argues that any sort of woman, with any sort of personality, will be more pleasant if she is well bred.

For example, Defoe argues that an intelligent woman who is not well bred will be "impertinent and talkative."  A well bred intelligent woman will know when to speak up and when to remain silent.  If a woman naturally has a bad temper and she is not well bred, she will be "haughty, insolent, and loud."  A well bred woman will be educated enough to know how to control her temper.  In both cases, the woman's education allows her to know how to act in ways that make her a more pleasant companion.

This, to Defoe, is the difference that "breeding" makes.  A well bred woman will be more pleasant to be around than one who is not.