John Calvin Commentary Genesis 1:27

"And God created man in his own image, in the image of God created he him; male and female created he them." — Genesis 1:27 (ASV)

So God created man. The reiterated mention of the image of God is not a vain repetition. For it is a remarkable instance of the Divine goodness that can never be sufficiently proclaimed. And, at the same time, he admonishes us concerning the excellence from which we have fallen, so that he may awaken in us the desire for its recovery.

When he soon after adds that God created them male and female, he commends to us that conjugal bond by which human society is cherished. For this phrasing, God created man, male and female created he them, has the same meaning as if he had said that the man himself was incomplete. Under these circumstances, the woman was added to him as a companion so that they both might be one, as he expresses it more clearly in the second chapter.

Malachi also means the same thing when he relates (Genesis 2:15) that one man was created by God, while he nevertheless possessed the fullness of the Spirit. For Malachi there discusses conjugal fidelity, which the Jews were violating by their polygamy. For the purpose of correcting this fault, he calls that pair, consisting of man and woman, whom God in the beginning had joined together, one man, so that everyone might learn to be content with his own wife.