I'm not well-acquainted enough with any of the other social aspects of the Catholic church--I can argue their dogma and their compulsive protecting of child molesters all day, but I don't really read much else about what goes on on the hush-hush in the church. So I can't really say whether or not there's a lot of surreptitious clergy-boinking going on.
I will say that it wouldn't surprise me. I'm sure a lot of the priests and monks and nuns and whoever else in the church are perfectly capable of the requisite self-control to live a completely celibate life--or have at least been brainwashed sufficiently into thinking sex is too evil to attempt--but not everybody can do that. Religious people are no different. I've heard second- and third-hand stories of women having affairs with priests and such, but I can't substantiate those so I won;t put them forward as evidence. If this kind of thing DOES happen routinely, I still expect it's something the male clergy members do but not the nuns--simply because there's less risk. A priest isn't going to get pregnant himself, and if they impregnate someone else the shame of unwed pregnancy AND fucking a priest is probably powerful enough to keep the impregnated girls and women largely quiet on the resulting baby's paternity. Nuns, however, CAN get pregnant and they wouldn't be able to shift the blame or hide this fact, so I wouldn't think they would be willing to take a risk like that, especially considering how vehemently they argue against any kind of birth control.
But like I said, i don;t know for sure so I'm not the person to ask.