The theme of Philip Larkin's "This Be the Verse" is best summed up by the first line of the poem's final stanza:

Man hands on misery to man.

More specifically, the theme is the way in which people are damaged by their parents and pass this damage on to their children. Larkin is not concerned here with evil people who want to inflict harm on their children. They might at least be dismissed as an aberration. His point is that parents warp the characters of their children and make them unhappy without meaning to do so. This unintentional abuse comes from their own childhoods, and they cannot help passing it on, with interest, to the next generation.

This is a situation which, the poet gloomily asserts, will never improve. The only way to break the cycle of abuse is to

get out as early as you can,

And don’t have any kids yourself.