The song has a different existence to the poem. This is also true for the original interpretation: the presence of the tune means that the perceptions of the listener will be different, will be moved or influenced in a particular direction that may or may not have occurred to a reader of the poem. The purist must then object equally to any setting.

I grant you that it seems (from this thread) that the difference between the versions is more than the usual "genetic drift", but changing the object of love from a girl praised by a boy, to a boy praised by a girl, does not seem to obviated all context or meaning.

And if it had: well, we then have a different song, from the same inspiration. I repeat: the original remains. The poem, and the song. We are all enriched - even if the latest addition falls below the standard of the earlier.