CG, those are terms used in Moral Theology.
“Perfect Sodomy”, in essence, refers to completed sodomy, sodomy where the copulative act is completed.
“Imperfect Sodomy” referes to cases where anal intercourse is started, but the act is not completed.
What exactly is your issue with Fr. Heribert Jone’s “Moral Theology”. It was the world wide standard Seminary text book on the sinfullness of individual action for the early to mid 20th Century?
It is indexed by particular act and then describes the sinfullness, or non sinfullness of the act, along with any mitigating factors.
I am certainly NOT a proponet of doing what was described.
But the fact remains, if someone went into the confessional during the 20’s to 60’s just about anywhere in the world and confessed the act described under 'imperfect sodomy" with their spouse; the priest, if he did not remember it from his Moral Theology class, would have looked it up in his in confessional copy of Jone’s and told the penitent what they did was not a sin.
While I personally think that anal penetration as an act of foreplay is abosolutely, positively disgusting and unhygenic, I have to concur that the Church does not find it an act against Natural Law.