Sonnet 20 is a sonnet, composed of three quatrains(four lines each) and a couplet with a kissed rhyme closing. This structure is the typical Elisabethian  sonnet. 

This sonnet seeks the boundaries of male and female sexuality. It focuses on the nature of Fair Youth. As he reads, the poet describes the physical characteristics of a male, but nevertheless with a mixture of feminine charm. There are also references to sexuality and the nature of sexual attraction, of the subject faced at both sexes.

In the first quatrain the poet describes all those characteristics of the young boy, which come close to those of a woman, finding them purer and sweeter. Shakespeare says that the Fair Youth was created by Nature to be like a woman. But each of these attributes is devoid of the disadvantages found in a woman who has them: the kind heart is not as mobile as a woman's. 

In the second quatrain, however, the poet argues that Fair Youth attracts other men because of its feminine beauty and of course of women. 

In the last quatrain and  concluding couplet, it ends with the verb "prickt" recalls that "addition" between the young man's legs. Since Nature has adorned the boy with its reproductive organ for the amusement of women, it makes a distinction between the physical love between a man and a woman and the spiritual, platonic and non-physical love it has for Fair Youth.