Alice von Hildebrand: “It is precisely because the marital bed is sacred that one should approach acts within it with enormous reverence. Degrading and perverse sexual behavior-- even it is it done by a married couple, who do not practice contraception-- should be condemned, as an assault on human dignity. The “pornification” of marriage should be resisted as vigorously as the pornification of our culture.”
Saint Alphonsus Liguori asks: “Whether it is a mortal sin for the husband to begin copulating in a disordered orifice, then afterward consummate the act in the proper orifice?”
His answer is, Yes, it is a mortal sin: “The reason is that this manner of his sexual act even without climax is truly sodomy, whether or not it is consummated, just as an act of copulation in the natural orifice of another woman is truly fornication, even if there is no climax.” [Moral Theology, Book 6, Q. 916]
Saints Aquinas and Augustine also condemn unnatural sexual acts in marriage.
[Hebrews]
{13:4} May marriage be honorable in every way, and may the marriage bed be immaculate. For God will judge fornicators and adulterers.