I guess that I should not be so reactionary, but we selected this title for my "Love and Responsibility" Week talk in contrast to another program called "Sex in the 21st Century." I don't think that sex changes all that much ;-)
Anyhow, the timeless and natural reality of sex is that children can result from it. As a matter of fact, there is no other natural way to have children. To engage in the act from which procreation can flow, it is necessary for an individual to surrender his autonomy and enter into communion with a complementary human being for the long term. One has to say: "I am not enough" to procreate and raise a child. It takes another, and it takes another who has what I do not have. There is risk in this, but there is so much hope! It is the natural "yes" to communion.
The marital commitment, including the marital act, is thus the natural basis for communion. No wonder it is under attack in a society that values radical autonomy above all else. I fear that is what sex in the 21st century is all about: destroying communion in order to preserve autonomy.