I was objecting to your statement that "There is no guarantee that a guy would stay and form a family, especially in these modern times."  I wasn't sure if you were implying the idea that all women and only women are good parents, which is a sexist and harmful stereotype.  Libertarians and society may be divided about abortion, but there are still women who will pursue the option when its available.  Most divorces are initiated by women.

Speaking of abortion and relative advantage, what do you think of the fact that under our current code of law, women are given all the power in this decision?  If a woman decides to have an abortion, the father can't do anything about it.  If a woman decides to keep the child, the father can't do anything and may have to pay child support.  Is this the kind of solution to the problem that you would support?

I reject the premise that childbearing is a service to "society".  It is a service to the biological parents, because it fulfills a biological urge, and to the child's family, because it provides a possible continuance of their culture and values.  I have no vested interest in the children of people who are unrelated to me biologically and culturally.  They might grow up to contribute to society in the form of labor, but if there's a labor shortage more can always be imported.