There is nothing easy about keeping a pedophilia relationship secret.
Personal experience has shown that a priest who desires to have sex with an adult woman will have very little trouble finding a willing partner. In fact, a heterosexual relationship with a consensual woman is the lowest risk violation of a vow of chastity, for many reasons.
In the history of human sexuality, it has been shown time and again, the only way to prevent a human from seeking out the sexual partner of choice, women, boys, sheep, etc, is to make it physically impossible. Threat of consequences has never been an effective deterrent.
The vast majority of priest assaults on kids are still secrets, so its not so hard to keep those secret. With an adult relationship, the moment the adult doesn't want to have that relationship they are likely to tell others that they had sex with the priest. They didn't break any vows. So they have no reason to keep it a secret. The kids have fear of authority that keeps them quiet, plus (unfortunately) a strong sense of shame and guilt. They were involved in "shameful" event that will bring them unwanted attention if they make it public. Even when they become an adult, they often don't want to be categorized as someone that was molested, so they keep it hidden.
Regardless, what matters is what the priest believes is true about being able to keep kids quiet vs. an adult. While things have changed alot in the past decade, for centuries they were able to keep kids quiet about it.
Consequences most certainly have massive impact on sexual behavior and choice of partner. Countless homosexuals lived their lives having sex with a gender they had no interest in, solely due to consequences. Countless heteros have married and had sex their whole lives with people they were not attracted to, because of social coercion to marry for reasons other than attraction. And as too prevalent as molestation is, it would be far moreso if there were no consequences for it, as their would be infinitely more infidelity in all types of relationships.