It doesn't appear that pre-ejaculate is the issue...

...but rather sperm that is in the actual ejaculate can be reintroduced to the vagina if penetration continues after withdrawal and ejaculation. Looks like this article covers it pretty well.…

Moms Expertise
    8Theresa Gould
    It does seem to cover this topic well. Thanks for sharing.
    About Tish
    Born: Homestead, Pennsylvania
    Current: State College, Pennsylvania
    Birth: March 04
    On since: Dec 17, 2013
    Working mom of two, happily married, editor/content manager of scientific publications for a scholarly society in Washington DC, attached parent, extended nurser. Parenting and loving the best way I know how!