Never allow grammer to set your morality. Never allow definitions to do so. This is the error of the British School of Philosophy, which concentrates on definitions and logic but ignores the natural law. The poet may be referring to the algebra of negative numbers, but in reality a wrong done to recompense a wrong is not making things right, but starting a spiral of revenge while calling it virtue.
The children when they read this only justify evil if they are monsters. The poet continuing to obsess over a woman rejected may indeed be making him monstrous. Not all poetry is to be emulated. The bad and dangerous to know also exist, and as a warning.
O grammer-rules, O now your vertues show;
So children still reade you with awfull eyes,
As my young doue may, in your precepts wise,
Her graunt to me by her owne vertue know:
For late, with heart most hie, with eyes most lowe,
I crau'd the thing which euer she denies;
Shee, lightning loue, displaying Venus skies,
Least once should not be heard, twise said, No, no.
Sing then, my Muse, now Io Pæn sing;
Heau'ns enuy not at my high triumphing,
But grammers force with sweete successe confirme:
For grammer says, (O this, deare Stella , say,)
For grammer sayes, (to grammer who sayes nay?)
That in one speech two negatiues affirme!
Sir Philip Sydney
Sydney is deluding himself. Young man, if she says to your love "No, No", get gone, get gone. There are many women, and the love you will find – early or late – requires mutuality. Not troll reasoning. For your emotions will flux. This is why we wed, and become one flesh: infatuation will fate but the life built entwined together withstands any storm.
Stella broke her vows and loved another: but that age was sane, and for that she was shunned. Two negatives do not become a positive, and calling evil good by some alchemy of Hegelian synthesis does not make it so. In this poem, Sydney is a warning. In life he did his duty. He left, and then, some time later, wooed and wed another. Let us follow that example.