Shakespeare's Sonnet 151 is one of his most difficult sonnets. The meaning and logic of the poem are unclear even to scholars. It is also one of the most bawdy poems, not just of the sequence, but in the entire corpus of canonical English literature. As a result, it is hard to explain without some frank terminology. Be warned: this answer has some NSFW content and swear words. If you are easily offended, please do not read beyond the horizontal rule.
The couplet can be paraphrased as follows:
Even though I call the woman who arouses and satisfies my lust "Love," do not assume that I have no conscience.
The lines have a surface meaning that is fairly easy to understand through this paraphrase: just because the speaker is in an illicit sexual relationship, it doesn't follow that he is without conscience. Or to put it differently, the sexual nature of the relationship doesn't preclude its being true love. And vice-versa: its being true love doesn't preclude a sexual relationship.
The rest of this answer is R-rated.
This harmony between sex and love might seem obvious to us in the 21st century. But until fairly recently, men would maintain a misogynist distinction between girls you fuck and girls you marry, with the former deemed "loose." Women with whom men had a sexual relationship outside marriage were not considered worthy of love and marriage—they were just vehicles for satisfying lust. The persistence of the game Fuck, Marry, Kill demonstrates that this split between lust and love is not entirely a thing of the past.
Particularly in the context of the 16th C., then, Shakespeare is making quite a radical claim by saying that lust and love are not in opposition, and that it's possible to have a relationship that's both lustful and in keeping with the dictates of one's conscience. Considering that in Christian belief lust is one of the Seven Deadly Sins, its being compatible with conscience is quite a claim.
But the couplet is even more complicated than that. A fuller explanation involves disentangling the intricate relationship between love and conscience that the sonnet sets up from the very first line, Love is too young to know what conscience is. This line deploys various puns on both love and conscience.
- Love is Cupid, the god of love, represented as a baby. Cupid shoots his arrows indiscriminately; being a baby, he has no sense of how to weigh right and wrong in his actions, i.e., he is without conscience.
- Love simply means romantic love as we usually understand the term. Romantic love too is a force which overpowers conscience.
- Love is a euphemism for lust. Shakespeare often blurs the distinction between the the two, for example in Romeo and Juliet with a pun on prick:
Is love a tender thing? it is too rough,
Too rude, too boisterous, and it pricks like thorn. (I.iv.25–26)
- Conscience means moral awareness, as we typically understand the term.
- Conscience also has a bawdy meaning: "experience or knowledge of sex." The word con is French slang for a woman's private parts, and science, which etymologically means having knowledge. In the Oxford Shakespeare edition of Shakespeare's poems, Colin Burrows has a note to this line that says:
The French con or "cunt" might be heard in the word,
giving a quibble on "carnal knowledge". (p. 682)
The previous sonnets have related that the Dark Lady, the addressee of this sonnet, has cheated on the speaker with someone else. Although this act of betrayal would be against conscience, it can be explained away because in matters of love or lust, conscience in the sense of moral awareness is irrelevant; it is pertinent only in the sense of sexual experience.
Through such puns, this first line sets up multiple contradictory meanings of both "love" and "conscience." The entire poem switches back and forth between the contradictory meanings, so that each line and each verse can be interpreted in multiple ways. A thorough explanation can be found on the shakespeares-sonnets site.
The closing couplet brings home these different meanings in a rich and playful way:
No want of conscience hold it that I call
Her love, for whose dear love I rise and fall.
The speaker has already mentioned his flesh ... rising at thy name. Rise and fall also have an obvious sexual connotation. So there is an implied pun on the prick of conscience. Using this bawdy sense, the speaker is saying:
Just because I call this woman Love, it doesn't mean that I am a virgin. She arouses me physically and I have known her.
This reinforces the surface meaning discussed above which uses the straightforward sense of conscience:
Even though this woman causes me to tumesce (by arousing me) and detumesce (after the sex act is completed), this lust does not preclude my love for her being morally justified.
Rise and fall also has a non-sexual meaning: like a soldier, the speaker will rise up to defend the woman he loves, and will die to protect her if necessary. These words participate in Shakespeare's layering of various meanings, some ordinary and some bawdy, to provide a many-sided perspective on his love.
To sum up: The couplet exploits polysemy to make a complex, witty statement about the nature of his relationship with the Dark Lady. It is the climax to a poem that undoes conventional distinctions between lust and love, or morality and sinfulness. The difficulty of the poem underscores the impossibility of summing up this complicated relationship in an easy, facile, or glib way.
- Shakespeare, William. The Complete Sonnets and Poems. Ed. Colin Burrow. The Oxford Shakespeare, general ed. Stanley Wells. Oxford World Classics. Oxford: Oxford UP, 2002.