First of all, that will be an amazing story. Depending on the religion and denomination of your "church man" (I assume he will be Christian since you are referring to a church), you will be able to play on very raw emotions of guilt, repressed desire, need and self-immolation that will highly enhance the sensuality of your sex scenes.
Where I am concerned, art shouldn't be leashed up. If in literature there is a genre called erotica that permits the full focus of a whole book to be on sexual relationships, then I do not see why some tastefully (your drawings seem very tasteful for what I have seen) drawn sexual scenes would be any less acceptable.
Rather than wondering how far you are willing to go where the depiction of the act itself is concerned, I would rather think about how to render it sensitive taking into account the victims of sexual abuse at the hands of religious leaders of different religions and denominations. I assume you want to go for the Roman Catholic priest (as opposed to a Protestant preacher, a Orthodox Christian or Greek Catholic Priest) since it is expected he has pledged a vow of celibacy. At the same time, sexual abuse at the hands of Roman Catholic priests has been very mediatized and your idea could bring back bad memories to people or ruffle some feathers.
You seem to be a kind, sensitive person, so I would tell you to go as far as your moral compass permits you but to keep in mind that some people will maybe need for you to add a disclaimer and to offer some emotional support.
I wish you a lot of luck with your work! I am sure it will be great!