Many elements in the poem indicate that the speaker is urging his beloved to seize the day. Look, for example, at the first two lines:
"Had we but world enough, and time,
This coyness, lady, were no crime."
This says IF we had all the time, you could be coy.
Then look at the start of the second stanza:
"But at my back I always hear
Time's winged chariot hurrying near…"
But we don’t have all the time, he's saying. Not only is our time limited, he feels and hears it rushing on. Therefore, the third stanza says, let's make love while we're still alive.