Yes, a lot of evidence. Exhibit number one is the Liber Gomorrhianus of Peter Damian published in the mid 11^th century (the Book of Gomorrah). Peter Damian was a very influential member of inner papal circles during the period known now as the 11^th century Gregorian Reforms, a period which saw a lot of Papal and clerical energy directed at reforming what was viewed as the lax morals and heresies of clergy including simony, lay investiture and clerical marriage. This was part of the same movement which led to schisms and conflict between papacy and monarchs, particularly with the investiture conflict with the emperors of the Holy Roman Empire, starting with Henry IV.

Damian's tract takes a theological stance against sins of homosexuality, but in particular sets out to explain the sinful nature of sexuality and homosexuality in the context of monastic life. He proceeds to be very detailed about the nature of the sins of monks (as well as other clergy, including females), and the order of their sinfulness. For example, Damian ranks the sins:

Four types of this form of criminal wickedness can be distinguished in an effort to show you the totality of the whole matter in an orderly way:

some sin with themselves alone [masturbation];

some by the hands of others [mutual masturbation];

others between the thighs [interfemoral intercourse];

and finally, others commit the complete act against nature [anal intercourse].

The ascending gradation among these is such that the last mentioned are judged to be more serious that the preceding. Indeed a greater penance is imposed on those who fall with others than those who defile only themselves; and those who complete the act are to be judged more severely than those who are defiled through femoral fornication. The devil's artful fraud devises these degrees of failing into ruin such that the higher the level the unfortunate soul reaches in them, the deeper it sinks in the depths of hell's pit.

Liber Gomorrhianus is not alone in identifying sex among clergy, it was preceded in the 10^th century by Regino of Prüm's De synodalibus causis et disciplinis ecclesiasticis and earlier in the 11^th century by Burchard of Worms' massive Decretum. However, where Regio and Burchard identify sex among clergy as sinful and prescribe penance for it, Damian is viewed as an outright obsessive attack. Damian was an advocate of deeply ascetic clerical practice.

Damian in his Liber Gomorrhianus identifies other issues of sex among clergy, including sex with minors. It was Damian's belief, and many of his fellow reformer's belief, that sex and marriage among clergy was a pollution of the sanctity of holy orders, a sanctity which they believed was corrupted in the centuries since Peter and the Church Fathers (like Augustine) set down the model of Christ's apostolic life.

Although Damian was one of the most important reformists with Hildebrandt and Humbert, and published important canonical works like Liber Gratissimus, his tract on homosexuality should be taken as evidence of the existence of sex in the monastery, but given the 'cool reception' given to the book by the papacy it can't been seen as canonical Church policy - we are told that Pope Adrian II took it from Damian and attempted remove it from circulation. Sex within the monastery was complained about through the balance of the middle ages, although not nearly so forcefully nor obsessively.

The full English translation can be read here:

• Peter Damian, transl by Peter J. Payer, Book of Gomorrah: An Eleventh-Century Treatise Against Clerical Homosexual Practices (Wilfrid Laurier University, 1982)

For background see:

• James Brundage, Law, Sex, and Christian Society in Medieval Europe (University of Chicago Press, 1987, 1990)