It is perfectly clear that no one can come near the purity of the Divine Being who has not first himself become such; he must therefore place between himself and the pleasures of the senses a high strong wall of separation, so that in this his approach to the Deity the purity of his own heart may not become soiled again. Such an impregnable wall will be found in a complete estrangement from everything wherein passion operates.
Now pleasure is one in kind, as we learn from the experts; as water parted into various channels from one single fountain, it spreads itself over the pleasure-lover through the various avenues of the senses; so that it has been on his heart that the man, who through any one particular sensation succumbs to the resulting pleasure, has received a wound from that sensation. This accords with the teaching given from the Divine lips, that “he who has satisfied the lust of the eyes has received the mischief already in his heart”; for I take it that our Lord was speaking in that particular example of any of the senses; so that we might well carry on His saying, and add, “He who has heard, to lust after,” and what follows, “He who has touched to lust after,” “He who has lowered any faculty within us to the service of pleasure, has sinned in his heart.”
To prevent this, then, we want to apply to our own lives that rule of all temperance, never to let the mind dwell on anything wherein pleasure's bait is hid; but above all to be specially watchful against the pleasure of taste. For that seems in a way the most deeply rooted, and to be the mother as it were of all forbidden enjoyment. The pleasures of eating and drinking, leading to boundless excess, inflict upon the body the doom of the most dreadful sufferings; for over-indulgence is the parent of most of the painful diseases. To secure for the body a continuous tranquillity, unstirred by the pains of surfeit, we must make up our minds to a more sparing regimen, and constitute the need of it on each occasion not the pleasure of it, as the measure and limit of our indulgence. If the sweetness will nevertheless mingle itself with the satisfaction of the need (for hunger knows how to sweeten everything, and by the vehemence of appetite she gives the zest of pleasure to every discoverable supply of the need), we must not because of the resulting enjoyment reject the satisfaction, nor yet make this latter our leading aim. In everything we must select the expedient quantity, and leave untouched what merely feasts the senses.
Source: On Virginity (New Advent)