As you point out, the final word is as remakable metrically as it is puzzling semantically. My own take on it (for what it’s worth) has been that Nature is made modest by the fresh clothing of Spring, while the alienated poet receives no such covering, and is therefore immodest. This is not as far-fetched as it sounds at first, when you think how often here is deciduous England the falling leaves of Autumn are associated with becoming naked:

Nature, in awe of him,
Had doffed her gaudy trim,
With her great Master so to sympathize:
It was no season then for her
To wanton with the sun, her lusty paramour.

Only with speeches fair
She wooes the gentle air,
To hide her guilty front with innocent snow;
And on her naked shame,
Pollute with sinful blame,
The saintly veil of maiden-white to throw;
Confounded, that her Maker’s eyes
Should look so near upon her foul deformities.

(from Milton). In others words, as Christmas approaches, Nature becomes naked like the infant Jesus, but covers herself with snow to hide the “foul deformities” of her twisted tree branches.

As you say, Larkin’s theme goes back a long way, but I think it’s as much about love as the intellect. Spring is the time for falling in love, but love does not come for the poet (Petrarch’s sonnet “zefiro torna”, endlessly imitated), or perhaps, Spring is the time for falling in love, let’s hope the poet gets lucky (Milton’s sonnet “O nightingale”, for example). Here, I think the poet sees such feelings as being behind him.

Anyway, what I like about Larkin’s poem is the unpretentious urban simplicity of his experience, more pigeons than nightingales.