Marriage - Avoid Misuse

Yes, there are boundaries to sexual acts from a moral perspective within marriage. When it comes to marriage, the unitive and the procreative aspects are equally important:

Sexuality becomes truly human when it is integrated into the total relationship of one person to another, in the complete and lifelong mutual gift of a man and a woman. Pope John Paul II wrote: “Only the chaste man and the chaste woman are capable of true love.” This means that married people are also called to be chaste if they are to truly love each other.

Married people living chastely can have vibrant sex lives. In the relationship between a man and a woman, chastity helps them love each other as persons rather than make each other an object of pleasure or satisfaction. Despite what the media and Hollywood suggest, the value of sexual intercourse does not lie in recreation, or physical gratification.

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It's a pity there's no direct link to the letter itself, only a few quotes. But even those quotes make a strong point. Here's another essay on the real meaning of sex:

Many people have the impression that the Catholic Church thinks sex is bad. This belief may have been reinforced by some over-reactions by members of the Church to the abuses of sexuality that have occurred in the last 50 years. However, the Church actually teaches that sexual union, when properly understood, is very good. It is sacred, beautiful and glorious!
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Every human being has an inner urge to love and to be loved. God created us that way. This urge can only be entirely fulfilled by union with God. During our life on earth, this urge leads us naturally to marriage and to sexual union with our spouse, which is meant to be a foreshadowing of the ultimate union with God in eternity. However, although marriage is beautiful and holy, the couple must realize that they are not the ultimate fulfilment for each other. Only God can fully satisfy the deepest longings of the human heart.
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When considering whether a particular sexual act is morally correct or not, Christians only need to ask themselves: Would this act be a reflection of God’s free, total, faithful, and fruitful love? “Masturbation, fornication, adultery, intentionally sterilized sex, homosexual acts, etc. – none of these image God’s free, total, faithful, and fruitful love. None of these behaviors express and renew wedding vows. They aren’t marital. Does this mean people who behave in such ways are ‘inherently evil?’ No. They’re just confused about how to satisfy their genuine desires for love.” (1)

It is also important to note that marriage is not a license that makes all sexual behavior acceptable. Not all instances of sexual intercourse between a married couple are truly “marital” in the sense of imaging God’s love. A married couple can still fall into the deception of lust and use each other for their personal gratification. This would not be authentic marital love. The same could be said of the use of contraception. As bluntly explained by Christopher West: “When a couple uses contraception, the spouses are really saying that they prefer the momentary pleasure of a sterilized orgasm over the possibility of participating in the inner life of the Trinity” (2).

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Sure, many are not going to like this kind of message. But let's face it - these principles make a lot more sense that the idea that sex is all about pleasure - for oneself first and only then - for the partner.