Essential to the survival to any society is the family. In our times it is, of course, under an attack on many levels. It is all the more essential to ponder what the family is, what it should be, and the particular threats that must be combated.
In pursuing these things, it is always splendid to recall or discover the counsels of the wise on the subject. Sacred Scripture, especially the counsel of our Divine Lord and the letters of St. Paul, give us the most powerful advice -- the inspired Word of God could do no less.
Of the writings of the early fathers of the Church, preeminent are the writings of the Patriarch of Constantinople, and Doctor of the Church, St. John Chrysostom (+407AD). For example, his commentary on Ephesians, Chapter 5: http://www.newadvent.org/fathers/230120.htm
More recently, the Popes have clearly echoed the constant Christian teaching in their encyclical letters and other addresses. Worthy of note is the great encyclical letter of Pope Pius XI, Casti Connubii, On Christian Marriage (1930AD): http://www.papalencyclicals.net/Pius11/P11CASTI.HTM
Pope Pius XI's essential reading was a follow-up on the earlier letter by Pope Leo XIII, Arcanum, On Christian Marriage (1880AD): http://www.papalencyclicals.net/Leo13/l13cmr.htm
Most recently, Pope Benedict XVI, on 9 March 2012, addressed a group of American Bishops on the very subject: http://www.vatican.va/holy_father/benedict_xvi/speeches/2012/march/documents/hf_ben-xvi_spe_20120309_us-bishops_en.html
In this address, he specifically mentions a contemporary American issue:
"In this regard, particular mention must be made of the powerful political and
cultural currents seeking to alter the legal definition of marriage. The
Church’s conscientious effort to resist this pressure calls for a reasoned
defense of marriage as a natural institution consisting of a specific communion
of persons...Defending the institution of marriage as a social
reality is ultimately a question of justice, since it entails safeguarding the
good of the entire human community and the rights of parents and children alike."
The Congregation of the Doctrine of the Faith actually commented further on this issue in 2003, under Bl. John Paul II, in a document entitled Considerations Regarding Proposals to Give Legal Recoginition to Unions between Homosexual Persons, signed by Prefect Cardinal Ratzinger: http://www.vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_20030731_homosexual-unions_en.html
Finally, in pondering this important issue raised by the Holy Father, it might be worth noting and reading this article that defends the traditional natural law position using only secular arguments, The Secular Case Against Gay Marriage by Adam Kolasinski:
from the American College of Pediatricians, Homosexual Parenting: Is It Time For Change?:
and, finally, this on certain statistical differences between homosexual and heterosexual households from the Family Research Council: http://www.frc.org/get.cfm?i=IS04C02
Marriage of the Virgin, by Giotto (ca. 1300AD)