Joaquin Navaro-Valls (sp?) said something very interesting recently, that I read in "Our Sunday Visitor". He said this could be grounds to possibly annul the ordination. I don't know what this really means theologically, but you know the Vatican is looking into something.
The reason why a marriage can be annulled is because the ministers of the sacrament of marriage are not the priest (who merely stands as the Church's witness) but the two baptized Christians who contract the marriage by their vows.
If one of them never truly intended to enter into a Christian marriage, then there never was a marriage and the sacrament never took place.
In the case of ordination, the minister is the bishop and the ordinand is merely a recipient of the sacrament, not a minister. What the bishop accomplishes in the sacrament is ex opere operato, indelible, and cannot be undone
If the ordinand is not a valid candidate (i.e. mentally disfigured by perversion, in this case) then he is guilty of fraud and the Church is justified in removing the benefits of the sacrament from him, including any rights, dignities and duties accruing to the priesthood, without appeal.
I assume he means the same thing as the annulment of the Sacrament of Matrimony. In either case, if the party has not entered into the Sacrament with the FULL INTENTION of living out the Sacrament, then the Sacrament was not valid to begin with.
In other words, if a man who is a pedophile or actively homosexual is ordained, but he has no intention of ever giving up that activity it would be as if the Sacrament had never taken place because God's grace would not be forthcoming to him. Same thing with a man or woman who marries, but has no intention of ever being faithful to his or her spouse; the vows that are taken are meaningless and therefore the Sacrament is not valid.