Now let us sport us while we may (line 37)
The speaker seems to say: we might not have all the time in the world, but we are still free to play. Sport and games are often associated with freedom. During World War II, one way that Americans show their support for the war is by indulging in their freedom to play baseball. The speaker, of course, is talking about sexual sport as freedom, or perhaps sexual freedom as sport.
Thorough [through] the iron gates of life (line 44)
Here, Marvell combines both freedom and confinement in the same line. He reminds us of the "crime" of the first stanza. It also states quite plainly that the speaker thinks life is a prison to escape. The speaker finally describes what he wants to happen – he wants to burst through the "iron gates" of the mistress’s "coyness." He wants to transform life into a free place.