I have read from cover to cover Pope John Paul II masterpiece book Theology Of The Body I still frequently use it for reference.
Nowhere does Pope John Paul II describe in any implied detail that Anal Sex is morally supported or to be freely practiced in sexual fore-play as a precursor to sexual intercourse in a Catholic Marriage sanctified by God who overlooks the entire life of married life between husband and wife.
Lets not mince words and call Anal Sex for what it truly is...quite plainly without secular rationalization and political correctness it is undoubtedly an act of Sodomy period.
One can read throughout several books in the Old Testament that Sodomy was a severe grievous Sin punishable by death.
In the Epistle of Saint Paul to the Romans
Douay Rheims version Chapter One verses 17 through 32 we read:
 For the justice of God is revealed therein, from faith unto faith, as it is written: The just man liveth by faith.  For the wrath of God is revealed from heaven against all ungodliness and injustice of those men that detain the truth of God in injustice:  Because that which is known of God is manifest in them. For God hath manifested it unto them.  For the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity: so that they are inexcusable.
 Because that, when they knew God, they have not glorified him as God, or given thanks; but became vain in their thoughts, and their foolish heart was darkened.  For professing themselves to be wise, they became fools.  And they changed the glory of the incorruptible God into the likeness of the image of a corruptible man, and of birds, and of fourfooted beasts, and of creeping things.  Wherefore God gave them up to the desires of their heart, unto uncleanness, to dishonour their own bodies among themselves.  Who changed the truth of God into a lie; and worshipped and served the creature rather than the Creator, who is blessed for ever. Amen.
 For this cause God delivered them up to shameful affections. For their women have changed the natural use into that use which is against nature.  And, in like manner, the men also, leaving the natural use of the women, have burned in their lusts one towards another, men with men working that which is filthy, and receiving in themselves the recompense which was due to their error.  And as they liked not to have God in their knowledge, God delivered them up to a reprobate sense, to do those things which are not convenient;  Being filled with all iniquity, malice, fornication, avarice, wickedness, full of envy, murder, contention, deceit, malignity, whisperers,  Detractors, hateful to God, contumelious, proud, haughty, inventors of evil things, disobedient to parents,
 "God delivered them up"... Not by being author of their sins, but by withdrawing his grace, and so permitting them, in punishment of their pride, to fall into those shameful sins.
 Foolish, dissolute, without affection, without fidelity, without mercy.  Who, having known the justice of God, did not understand that they who do such things, are worthy of death; and not only they that do them, but they also that consent to them that do them.
If Sodomy is morally wrong I'm at a loss to understand how Catholic married couples can comfortably and euphemistically diminish the demoralizing act of Sodomy and make it justified within sexual foreplay in Catholic Marriage. Something seriously does not jive here.
I keep hearing Catholics say over and over again that the Magisterium has never made an official ruling on Anal Sex/Sodomy as it being permissible in Marriage sexual foreplay.
The Church shouldn't have to spell it out and engrave it on our noses.
From a purely Scriptural standpoint; it is a gravely disordered SIN.
Catholic Married couples don't get dispensation to carry out the Act of Sodomy even if it is consensual.
I don't know how somebody can make this any clearer.