The chief device--which is an element of plot mechanism--employed in Shakespeare's Sonnet 20 is conflict. The poet loves the 'master-mistress' of his passion, but because, as the poem develops, we see that the person addressed is a young man, the poet-lover must love in vain - specifically, without physical cosummation. The womanliness of the young man - a (presumably) feminine face and a woman's 'gentle heart', moves the poet-lover to passion. Here we encounter another conflict: the idea that because the beloved is a man, he does not possess what the poet regards as the negative, less lovable traits of feminine nature: falsehood, guile, and infidelity. Instead the beloved is morally as well as physically desirable because of, not in spite of, his manhood.
Nature is personified as a female force who, in creating the perfect being, 'fell a-doting' and has decided that she will create a man rather than a woman, and by 'addition' - very obviously the man's penis - has thus defeated the lover in his desire.
The poem ends with a pun: a play on words in which we understand 'prick'd out' as a gardening term, meaning the process of preserving the best and strongest plants to grow and flourish, together with the more colloquial/bawdy sense of 'prick' to mean penis. Thus we are back with conflict: the young man's body is for women to enjoy, although the poet - 'mine be thy love' - assumes that the young man returns his affection, and there appears to be more than a hint of conflict in the desires and affections of the beloved.
There are a great many puns in this poem; puns on meaning, and puns we can hear. Puns are devices by which the poet loads apparently simple lines with layers of suggestion. Note the word 'hue' - a man in 'hue' (meaning colour or tint) may well simply refer to the young man's male aspect or appearance, but 'all hues in his controlling' would suggest the word in its sense of 'value' and 'form': the young man commands the desires of those women and men who desire him. While we do not know for certain how the word 'hue/hues' was pronounced in Shakespeare's time (especially in the matter of the pronounced 'h') but if you say the line aloud, you can probably hear 'use' - with a meaning here of sexual employment - or even 'yous'. (See also Sonnet 82, where use and hues figure prominently.) Similarly, consider 'nothing' as 'no thing' - ie, no penis: the lover's 'purpose' is a woman, who possesses 'no thing'.
This sonnet has been much debated as a clear admission of Shakespeare's homosexuality. While this, or at least his bi-sexuality (which seems more likely here) is probably the case, the conflict remains within the context of the poem itself: a courtly love poem which by tradition was addressed by a languishing lover to an unattainable lady: usually she was married. This sonnet enlarges on the older tradition - Shakespeare takes the ideal of the unattainable lady and, by making his subject male, makes him literally inaccessible.ShakesS