I disagree that such a thing as "homosexual orientation" exists. If they are celibate and have never given into the perverse temptation they struggle with, then they are not 'gay'. They are merely tempted. Does being tempted to steal make one a thief? Does being tempted to kill make one a murderer? Only giving into the temptation establishes the behavior. BTW, self identifying as 'gay' demonstrates that they have given into the temptation. They identify with their sin rather than with thier Savior
Temptation to do evil is not part of our divine nature, but is rather "concupiscence," an effect of evil in our lives. One who resists great temptation has borne great suffering, and is morally heroic, perhaps moreso than someone who has faced less temptation. But the persistance of a strong, unusual, and unnatural concupiscence is a troubling indicator that one may not be capable of effective ministry.
God gives us the graces of sanctifying natural urges and processes by making them means towards our salvation. What once had a holy end (sexual desire) has been perverted by the fall of man to an evil (lust), but can through surrender to God (contrition) can be rectified to a reflection of God's love (marriage). Thus, the very thing which makes us tend towards evil can become an instrument of our salvation. The priesthood, however, necessarily requires an abandonment of that tool for sanctification, and so, therefore, should be reserved for those whose sexuality has been less perverted.
It is true that all priests, like all people, struggle with concupisence, but homosexual desire is a form of concupiscence which is both extra dangerous for priests (since they are denied normal outlets for developing a healthy sexuality), dangerous for their flock (who depend on the priests to have a healthy sexuality, and can be gravely harmed by priestly abuse), and made particulalry more dangerous by the strains and sacrifices of the priesthood.
This is in contrast to some other concupiscence; a tendency towards greed may be helped the asceticism of certain religious orders.