It is from Astrophel and Stella, a fictional account of a love story - which might have some relevance to Sidney himself, but that is debate-able. "Astrophel" means star (astro) and lover (phil) and Stella refers to star (stellar). So, Astrophel is the star lover and Stella is his star. In this particular sonnet, the speaker is just asking if love, or more to the point, Desire is sinful and he concludes that if love is sinful, then call him a sinner. The idea he's toying with is this: if love and desire leads one to sinful thoughts, then is love sort of a primrose path to sinfulness?

"Love on me doth all his quiver spend" - Cupid's arrow has got his heart.

The speaker is talking to a critic: someone who is critical of his love. The speaker refutes the idea that his love, since it pains him, is sinful. Then he says, even if it is sinful, then "let me sinful be!" Speaker describes the critic's words with rhubarb (bitter, sour). Speaker is essentially defending his love, saying it is constant and pure: "Well stayed with truth in word and faith of deed."