A reasoned basis for this statement is probably too much to ask.
The following is a simple natural law argument for monogamy and post-marital sex.
1) Looking at the bodies of men and women, it's obvious that they play complementary roles in the process of bringing new human life into the world. This is the way nature meant children to come into the world, hence the term, "reproductive system" or "genitals."
2) No reasonable person would argue that children are better off raised by anyone other than their biological parents. Nevertheless, there are some unreasonable people who do. These people need to become acquainted with the data regarding the rate of sexual abuse by step-parents.
3) And if it's not obvious that children are generally better off being raised by both of their natural parents, then again, I would point them to the sociological data regarding the effect of divorce on children.
4) The act of sexual intercouse is pleasurable and ordered by nature toward procreation. It is also unitive, as common sense, common experience, and current scientific data affirms.
5) It follows then that this unitive aspect of intercourse is ordered by nature toward the unity of the man and woman for their benefit(4), and also for the benefit of the children who may issue from the act (2).
6) Moreover, nature intends this unitive bonding to be lifelong, since the children of parents bonded together for life are generally happier and healthier than children raised otherwise (3).
7) If one or the other members of the bonded couple engages in intercourse with a third person, then new natural bonds will be formed, weakening the original bond which nature intended, to the detriment of the original couple and their children (5).
8) Since nature intends intercourse to unify the couple for life, for the benefit of the couple and the benefit of children which may issue from the act, no man or woman should engage in intercourse without having made a lifetime commitment to the other partner.
8) Therefore, intercourse outside of a lifetime commitment is harmful and wrong.
Our reward is that we're still together after 23 years and our kids are well-adjusted and committed to saving themselves for marriage. And yes, we know other families just like us in this respect, even in 2007.
Everyone of us makes Planned Parenthood less relevant and less wealthy.