Who had sex with women to produce a race of giants?

The sons of God

The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. ...

There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. Genesis 6:2-4