Excerpt from:
Moses Maimonides, The Guide for the Perplexed, translated by Shlomo Pines. (University of Chicago, 1963)
Originally written ca. 1190.
Part III, Chapter 49, Page 609:
Similarly with regard to circumcision, one of the reasons for it is, in my opinion, the wish to bring about a decrease in sexual intercourse and a weakening of the organ in question, so that this activity be diminished and the organ be in as quiet a state as possible. It has been thought that circumcision perfects what is defective congenitally. This gave the possibility to everyone to raise an objection and to say: How can natural things be defective so that they need to be perfected from outside, all the more because we know how useful the foreskin is for that member? In fact this commandment has not been prescribed with a view to perfecting what is defective congenitally, but to perfecting what is defective morally.
The bodily pain caused to that member is the real purpose of circumcision. None of the activities necessary for the preservation of the individual is harmed thereby, nor is procreation rendered impossible, but violent concupiscence and lust that goes beyond what is needed are diminished. The fact that circumcision weakens the faculty of sexual excitement and sometimes perhaps diminishes the pleasure is indubitable. For if at birth this member has been made to bleed and has had its covering taken away from it, it must indubitably be weakened. The Sages, may their memory be blessed, have explicitly stated: It is hard for a woman with whom an uncircumcised man has had sexual intercourse to separate from him. In my opinion this is the strongest of the reasons for circumcision.
Page 611:
This class of commandments also includes the prohibition against mutilating the sexual organs of all the males of animals, which is based on the principle of righteous statutes and judgments, I mean the principle of keeping the mean in all matters; sexual intercourse should neither be excessively indulged, as we have mentioned, nor wholly abolished. Did He not command and say: Be fruitful and multiply? Accordingly this organ is weakened by means of circumcision, but not extirpated through excision. What is natural is left according to nature, but measures are taken against excess. He that is wounded in the stones or hath his privy member cut off is forbidden to marry a woman of Israel, for such cohabitation would be perverted and aimless. Such a marriage would likewise be a stumbling block for the woman and for him who seeks her out. This is very clear.