I am quoting sections from Bill Hybel's words on Evangelicals and Gender Equality
by Lynne and Bill Hybels:
First, the history from Bill's youth pastor role:
Our approach to leadership was pragmatic; it seemed obvious that we needed male leaders for the guys to relate to and female leaders for the girls to relate to. In actual experience, it was easier to find high school girls who were spiritually mature and skilled in leadership than it was to find guys. From a practical standpoint, it would have been unthinkable not to allow girls to lead.
Then, "In 1975, the high school youth ministry “birthed” a church for adults called Willow Creek Community Church." In this section Bill writes:
I have to confess that at that point I was not absolutely convinced theologically that including women in leadership was the right thing to do, but neither was I convinced that it was prohibited.
Then, when they started to get attention from both Christian and secular media, the question came (with my emphasis):
Did we have a rational defense for our position? In response, we commissioned our elders to do an intensive, eighteen-month scriptural study of the issue of women in leadership. I did not feel it was right to sideline the women whom God seemed to be using while we did this study, so we pursued a parallel track of study and continued observation of how God worked among us through the leadership efforts of both men and women.
Dr. Gilbert Bilezikian, a Wheaton College professor and Willow Creek elder, led the study. The conclusions of the study were published in 1985 in Bilezikian’s book, Beyond Sex Roles: What the Bible Says about a Woman’s Place in Church and Family. In summary, we concluded that before the Fall, men and women related to each other as co-regents, both bearing the image of God and called to join together in caring for the world he had created. Both men and women were responsible to fulfill their ministries of service for God’s glory in the manner God had gifted them and to the degree to which they had been apportioned faith. Tragically, in the Fall, this cooperative relationship between men and women was deeply wounded. We believe God’s gracious plan for redemption is that everything that was broken through sin—including the relationship between men and women—might be restored to the beauty that existed during the first days of Creation.
TL;DR: the Willow Creek elders concluded that, before the Fall, men and women were equal partners and so this should be the pattern within the church.