Humans are sexual animals. Celibacy and sexual abstinence go against our very nature, so it is hardly surprising that there is sexual activity among those groups, either consensual or coerced. One of the reasons for requiring abstinence from the religious is to test their dedication to their faith.
Those who are able to maintain sexual abstinence are considered to be more trustworthy, since they are turning their backs on a basic need of human beings. If they will submit to authority in that, they are likely to submit in many other ways. Those who fail in being abstinent have often been cast out of the organization in shame. However, failure to abstain is more common than we imagine, so adjustments are made in many cases to retain people in their orders.
The entire thing is hypocritical from the very start. Denying one's basic needs has become central to the Roman Catholic Church's basice doctrines for the religious. That contradicts the very nature of the human experience. So, it's a false doctrine, if you believe that an all-knowing deity created humans. Hypocrisy in one area tends to lead to hypocrisy in many other areas, as well.
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