Shakespeare's Sonnet 20 can be rather tricky, so let's take a look at what's going on in it. This sonnet is part of a series written in admiration of a young man. Nature has provided him with beauty like a woman's, and he even has a “woman's gentle heart.” While he has these positive characteristics of a woman, he does not have a changeable nature (which the speaker also attributes to women) or a falseness or insincerity. In fact, the youth is a “man in hue,” in shape and form, and this attracts all people to him. Men and women both look at him and admire him.
The speaker then speculates that perhaps nature created him as a woman first but then changed him to a man because she, too, began to dote upon him and love him. Nature is personified here and given the role of creating and shaping human beings. She is a fickle maker who falls in love with the youth she creates and then changes him to suit her pleasure. The speaker actually doesn't believe this account; he is trying to explain the beauty of this young man in a creative, metaphorical way.
The speaker clearly loves this youth and is attracted to him, yet he knows that while he may indeed love the young man, the latter is set apart “for women's pleasure.” This is expressed in the sonnet's final couplet. Shakespeare's couplets often express a turn of meaning in his sonnets, and this is no exception. The speaker resigns himself to loving the youth with a different kind of love, for nature has made her choice, and he can do nothing to change that.