A good example to begin with is St. Augustine [354-439], who, perhaps more than any one other individual, set the sexual standards for the Christian Church. Briefly, St. Augustine had converted to Manichaeanism as a young adult, and through he had striven for many years to become one of the Manichaean "Adepts", A person who among other things lived in chaste and celibate life, he had not been able to achieve this status. The difficulty was that though his spirit was willing, his flesh was weak, and Augustine, in spite of his belief system, was unwilling to give up sex, particularly with his mistress. He later reported that his constant prayer during this period of this live was "Give me chastity - but not yet" (Augustine, 1955, VII, vi, 17).
Unable to forego sexual activity, Augustine decided to avoid trying to be an Adept and to regularize his life by marrying. Once this decision was made, he sent his mistress and illegitimate son away and then set out to choose a bride. He selected a prepubertal girl, and since technically he could not marry her until she came of age (i.e., had her menarche), he was betroyed to her. Unable to give up sex even for this brief period, he took another mistress, and this act, among other things, brought on a personal crisis that led him to convert to Christianity and swear off sex for the rest of his life (Augustine, 1955, IV, ii, VI, xii, VII,i)
Obviously, Augustine was more than twice the age of his bride-to-be (Bullough and Brundage, 1983)
Bullough, V. and Brundage,J Sexual Practices and the Medieval Church Buffalo: Prometheus Books, 1983