When My Love Swears That She is Made of Truth

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 untutor’d youth,
Unlearnèd in the world’s false subtleties.
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 suppress’d.
But wherefore says she not she is unjust?
And wherefore say not I that I am old?
O, love’s best habit is in seeming trust,
And age in love loves not to have years told:
Therefore I lie with her and she with me,
And in our faults by lies we flatter’d be.

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Here is sort of a very rough modern English
"translation" and far less poetic way of saying
what The Bard said in this sonnet:

When my mistress swears that she's completely truthful
I believe her even though I know she's lying
So she'll think I'm a naive young man
Ignorant of the ways of the world and its lies
In my vanity, I think she considers me young
Even though she knows I'm past my prime
I openly agree with her lies
And both of us are hiding the truth
But why doesn't she admit she's a liar?
And why won't I admit that I'm old?
The best thing about love is found in trust
And older lovers don't want to speak of their age
So we both sleep together
And in our imperfection we flatter each other with our lies