Shakespeare's Sonnet 138, When my love swears that she is made of truth, is a dark, pun-filled discussion of a sexual relationship between a no longer youthful speaker and his faithless mistress. The general situation is stated in the opening quatrain, and the remainder of the poem gives specifics.
When my love swears that she is made of truth,
I do believe her, though I know she lies,
That she might think me some untutored youth,
Unlearned in the world's false subtleties.
The common double meaning of "lies" to recline and to speak a falsehood is clearly operative. Also the homonym of made/maid is strongly suggested. The speaker is aware that his love is not a "maid of truth." Still he prefers to accept her declarations though deep down he knows she is swearing falsely. He knows that he is neither a youth nor a man unschooled in the deceptions that are part of the fabric of everyday life.
Thus vainly thinking that she thinks me young,
Although she knows my days are past the best,
Simply I credit her false-speaking tongue:
On both sides thus is simple truth supprest.
The puns persist. "Vainly" has multiple meanings. In Shakespeare's time it meant foolishly. Cf. The Comedy of Errors "there is no man so vain/ That would refused so fair and offer'd chain." As used in this sonnet, the adverb also means "futilely" or "in vain ," as well as "to satisfy one's vanity, conceit or pride." Then "Simply" and "simple" carry both meanings of "stupid(ly)," "guileless(ly)," like a simpleton and "plain(ly) or without adornment. "I credit" means "I place belief in." Simply stated, he believes what he knows to be untrue.
Thus the lovers not only lie to (and with) each other but the man, at least, lies to himself. He realizes that his performance in bed betrays his slightly advanced age. He is clearly older than his mistress but not so very old that he would have no hope of passing for someone younger.
But wherefore says she not she is unjust?
And wherefore say not I that I am old?
Oh, love's best habit is in seeming trust,
And age in love loves not to have years told:
"Unjust" in this usage means unfaithful. The rhetorical question posed is "Why does she not admit to having other sexual partners?" The answer is that he does not want to hear it, though at a deeper level he knows the fact. His second question of the quatrain asks why he isn't honest about his age. In a more open, honest, and mature relationship there would be acceptance and free admission of faults. But the speaker opts for self-delusion and knows that is what he is doing. Love's "habit" punningly refers to a garment of bodily concealment along with the primary meaning of an accustomed practice. "Seeming" has as its primary meaning "apparent," but also suggests an appearance which is deceptive or illusory as opposed to real or genuine. "Told" means both "openly stated" and "counted."
Therefore I lie with her and she with me, And in our faults by lies we flattered be.
With the concluding couplet comes a sense of circularity and completion by returning to the pun on "lie" stated in the opening quatrain. They continue to sleep with each other because habits are hard to break. Better to enjoy the fruits of lovemaking and stifle knowledge that the woman's sworn oath is perjured and that the speaker is "over the hill."
As is nearly always the case with Shakespearean sonnets, the structure is three quatrains of predominantly iambic pentameter followed by a rhymed couplet. Thus the rhyme scheme is abab cdcd efef gg. Line 7 contains a variation that casts into prominence the word "Simply." The word is a trochee (an accented syllable followed by an unaccented syllable) and not the expected iamb. Its arresting quality then attaches to the following adjective "simple."
The multiple denotations of that adverb capture the speaker's attitude and aptly describe the entire sonnet. For him to accept as true that which is pleasant to hear despite its patent falsity is to be simple-minded. Only naive and simple readers will find the poem simple and without artifice and subtlety.