and not having children are foolish. The last line of the quatrain (line 8) emphasizes the harm that the young man is bringing on himself. The young man is an enemy or "foe" to himself. He is being cruel to himself. Here the speaker is indirectly suggesting that the young man would find life (or passion) more rewarding if he would experience passion with a woman. Then his passion could grow and not burn itself out.
In the third quatrain the speaker is more direct. But the words do suggest the passage of time. The speaker calls the young man "the world's fresh ornament" (line 9) and the herald (or messenger) of springtime (line 10). The speaker is emphasizing the youth of the young man. The word fresh suggests newness. The young man is currently like a new decoration or ornament. But new decorations eventually become old and faded and tarnished. They lose their shine, their luster, and their appeal. Similarly, the young man may be like the beginning of springtime. But eventually spring turns into summer, and the fall and winter seasons are not far behind. The "gaudy" spring, which suggests the luxuriant growth of flowers and leaves at that season, eventually becomes a dry and barren season, fall. The leaves come off the trees and the flowers all disappear. The speaker, then, is telling the young man that his youth and beauty will not last. Eventually they will disappear as well.
Words often have more than one meaning, and poets often use a word to mean more than one