It doesn't take much to be listed as a sex offender. I heard of one case the other day where two boys in their early teens had sex together. One of them admitted he was gay, and was listed as a sex offender. The other denied being gay, and was not listed. The one kid will have the label of sex offender following him all his life (unless the law changes), for consensual sexual activity with someone his own age.
So before answering whether or not a registered sex offender should be an ordained minister, I would want to know if we are talking something harmless that happened when he was a kid, or if we are talking pedophile or rapist.