Before paraphrasing the poem itself, I must first start with the title. The speaker is telling of his unending love to his "coy mistress." But at the time this poem was written, in the late 17th century, a mistress was not the lover of a married man. Instead, it refers to a woman who is loved and courted. Coy is defined as shy and modest. The speaker is telling his coy mistress that if there were enough time in the world, her shyness would be no crime because time would not be a factor. They could sit down and pass the days just loving each other. He (the speaker) would sit by the river Indian Ganges, and write love songs about her. He would love her for eternity, and she could always refuse him, because without time as a factor, there would be no rush. His love for her would grow slowly, slower than empires grow. He would praise her eyes for a hundred years, and praise each breast for two hundred years. He would spend thirty thousand years praising the rest of her body; he would dedicate an entire age to each part of her. Saving the best for last, he would spend an age talking of her heart. He loves her so much that she deserves all this praise, and he cannot imagine loving her any less than he does; he cannot imagine giving her any less praise than she deserves.
But this is not possible, because he knows that time is short; our lives are short indeed compared to the time it would take him to properly praise her. Because life is so short, because "Time's winged chariot hurr[ies] near" (line 22) she would be dead long before he got done praising her. She would no longer be beautiful, and she would not hear his songs of love, because she would be dead, buried underground. Before he could take her virginity, the worms would eat her. Her honor would turn to dust as she decomposes, just as his lust would turn to ash, because they will both be dead. There is humor in the statement "The grave's a fine and private place,/ But none, I think, do there .