Well, obviously, their primary purpose is to nurse infants. When one considers their design and placement, they are perfect for the job. But that isn't to say they aren't ornamental.
But their ornamental quality is only secondary and, frankly, quite fleeting. The ornament that is of great price in the sight of God, and in the sight of noble men, is the ornament of a meek and quiet spirit. When Lemuel's mother taught him what to look for in a woman, the entire description was of inward qualities with this conclusion:
[Charm] is deceitful, and beauty is vain: but a woman that feareth the LORD, she shall be praised.
"Beauty is vain." It is empty, and impotent. No man will love a woman because of her outward beauty. As her beauty fades, so will his interest in her. The only beauty that is imperishable is the beauty of holiness. And only a holy man can appreciate it.
Now most people will jump right on board with this view of beauty, especially a woman because she can feel most accutely the violation when one has simply used her to satisfy his lusts.
It's the same with sex. All the sensual qualities we so highly esteem are only secondary, fleeting and vain. They are powerless to generate any real oneness, intimacy or to express any kind of true love, but we tend to ascribe that power to it, I think, to justify our inordinate sexual apetites (apetites no doubt awakened by our immersion in an increasingly sensual and debauched society). We even use Paul's words in 1 Cor. 7 to coerce our partners to yield to our sexual desires on demand.
It's an unfortunate situation we find ourselves in really, and until we can learn to put sex in its proper perspective, we will never have a healthy view of marriage and the family.
Do you want to know what my wife considers the most intimate expression of my love for her? It wasn't, "Hubba, hubba!" It was when I expressed my true and deep desire to give her my heart and to be her friend in the full sense of the word. Tears welled in her eyes, and she said, "Let's be friends."