What does the church teach about spousal sodomy?  I loath this subject, but the clergy apparently have discussed and possibly addressed this, so I want to know the particulars.  

In denzinger, a footnote for dz 2239/2240 on the abuse of matrimony quotes a response from what appears to be the 1916 work "institutiones Alphonsianae" - declaring that a wife because of a threat of death or grave injury can cooperate in an interrupted copulation with her husband.  But, by no means can she do so, not even for the sake of avoiding death, in a sodomitic copulation.  

This begs the question.  Why would this explicitly teach that a spouse cannot cooperate in an interrupted sodomitic copulation?  Is it because the church was either implicitly approving when uninterrupted?  Was the church not condemning uninterrupted sodomitical copulation?  Why would a footnote in denzinger be useful?  Do you follow?  

Next is teaching from heribert jone moral theology.  It is from a 1962 reprint of an 18th century work(18th century was a long time ago).  According to the translation, it teaches in orwellian fashion that spousal sodomy is actually not sodomy nor a sin, and it implicitly teaches that there is a "perfect" type of sodomy.

This is from a michael hoffman link about heribert jone moral theology teaching about spousal sodomy - http://revisionistreview.blogspot.com/2013/06/another-challenge-to-hoffmans-thesis-on.html

“757. I. Imperfect sodomy, i.e. rectal intercourse is a grave sin where the seminal fluid is wasted.”

The Moral Theology manual clearly teaches, for those who can read, that when the husband has anal intercourse with his wife without the intention of ejaculating in her rectum, he is free to do so in that it is not sodomy and it is not a grave sin:

“Excluding the sodomitical intention it is neither sodomy nor a grave sin if intercourse is begun in a rectal manner with the intention of consummating it naturally...”

In other words, if the husband begins his marital act with his wife by having rectal intercourse with her, as long he concludes the act by ejaculating in her vagina, it is not sodomy and it is not a grave sin.

It is also not sodomy or a grave sin, “if some sodomitical action is posited without danger of pollution.”

[All quotations from p. 539 of Moral Theology (Newman Press, 1962) and bearing the Imprimatur and Nihil obstat.

The footnote is taken from 1954 denzinger thirteenth edition by loreto publishers.  If someone owns older versions, perhaps it is useful to search for that footnote.  

When there's mortal sin involved with venereal pleasure it's due to it not being within marriage and ordered therefore at least ultimately towards procreation.

So the principle is that it isn't the pleasure per se that's mortally sinful.

With that said, there can be varying degrees of venial sin involved when there's inordinate pleasure taken in the activity.

There does appear to be a slightly overlooked / missing element here regarding the fact that this seems contrary to nature (even if "perfect").

It doesn't hurt to post the rest of dz2303 #4.  I put bold on the part that some may use to argue to my contrary.  

"although a priori new methods cannot be excluded merely because they are new, nevertheless, as far as artificial fertilization is concerned, not only is there need of the greatest circuмspection, but it simply must be avoided.  By these words we do not necessarily forbid the use of artificial means, which are destined only either to render the natural act easier or to bring it about that the completed act attain its end in a natural way.

Let it not be forgotten: only the procreation of new life, which takes place according to the will and order of the creator, obtains to a truly perfect degree the ends intended by it.  Such procreation corresponds at once to the corporal and spiritual nature and the dignity of the spouses and to the normal and happy development of the infant. "    

Now, if someone could please provide a papal teaching/quote saying that sodomy is contrary to nature, then we can close this case and move on.

What do you think are some examples of "artificial means" spoken of here by pius xii?  Because, the more I read this passage, the more it sounds as if he is speaking about things of this nature.  Because, he talks about completing the act in a natural way.   "By these words we do not necessarily forbid the use of artificial means, which are destined only either to render the natural act easier or to bring it about that the completed act attain its end in a natural way.

Matto - brilliant.  I think you just nailed it.  If spousal sodomy is not a sin, then the spouse theoretically could not refuse.  She would have to render this marriage debt.  

I think the next step is to brush up on the subject of degrees of marital sin, obedience, and the marriage debt.  

If it is not a sin to commit acts of sodomy then it if the husband demands that his wife commit sodomy with him wouldn't she sin if she refused because the wife has a duty to obey her husband? So if it was not a sin then we would have the absurdity of women going to hell because they refused to be sodomites.

No, the marriage debt does not refer to any specific types of acts; there's no obligation to do something specific just because the spouse wants to do it.

Sodomy between two men would certainly be against nature.  Sodomy which is completed would certainly be against nature.  But would sodomy during activities which are completed in natural intercourse be contrary to nature?  In other words, is it the "Onanistic" aspect which would render it inherently contrary to nature?  Are there then other activities which normally considered contrary to nature but would not be contrary to nature if not completed in Onanism?  I for one do think that there's something inherently contrary to nature in the act itself in the fact that the excretory system was clearly not designed by God for this purpose.  Whether it's a mortal sin if not complete?  Not sure.

But I think that it's probably a waste of time for non-professional lay people like us to dwell on this.

Since many approved reputable theologians seem to hold this position, that it's not grave sin per se so long as the marital act reaches its completion in a natural way, a Catholic who does so while following the position of these theologians would certainly not be committing a grave sin (cf. St. Alphonsus' "probabilism" vis-à-vis moral theology).  That's not to say that such Catholics would not sin at least venially (and perhaps even with relatively-serious venial sin) due to excessive indulgence in pleasure.  That's not to say that such a thing would please God or would be compatible with seeking perfection.  But it's only to say that it wouldn't constitute grave sin.