What does the flea by john donne mean?

In 'The Flea', John Donne is trying to get a girl to sleep with him, saying that the flea is their marriage bed because it drank blood from both of them. He tells her not to kill it, but she does out of spite or to put an end to the conversation. He then tells her that killing the fle did not soil her and neither would pre-marital sex.