This man's lover is shy ("coy"), or pretending to be shy, and wants to take their relationship slowly. He, on the other hand, is ready for more. So, the poem is an attempt to seduce her with his words and to sweet-talk her into bed.
He says that if he had all the time in the world, he'd happily take things slowly. He'd love to spend hundreds of years praising her beauty - moving his gaze from her "forehead" down to her "breast" and then "the rest".
But time is limited. There's no point in waiting until they're dead in their graves - that's no place for kissing. He knows she wants it too ("thy willing soul"). So, he reasons that now is the time to "sport us while we may". They should "devour" time hungrily, not lie trapped in its slow-moving jaws ("languish in his slow-chapped power"). They can't stop the sun moving across the sky, but they can "make him run" by enjoying every moment to the full.