In most societies, in most of time, the status of women is defined by their unique ability to bear children. That ability extends beyond conception and childbirth. Bearing children doesn't begin to be encompassed wholly in those two acts; socially, and probably biologically, motherhood rests with us for life. And our children's lives too will embody our motherhood, whatever form it has taken. Including our absence.
Men who choose life partners in marriage who are men choose also relationships with the mother for any children born into such marriages. There are cultures where the inability to bear children (including those where no female is available in the correct marriage kinship category) is met with various solutions, concubinage being the most common and, once, widely accepted. Now even concubinage is displaced by an outright denial of any special natural link between mother and child in surrogacy. Surrogacy does not have the minimal social elaborations and protections afforded to concubinage.
Gay marriage strips away the socially-constructed but naturally-generated statuses that motherhood confers on women. Usually, and in all current same-sex marriage institutional arrangements, surrogate-birth women are denied any social role engendered by natural requirements. If surrogate-birth women do have any social statuses proffered to them these are entirely within the gift of the gay spouses. Payment extinguishes any rights in motherhood for both mother and child in gay marriage even if, occasionally and entirely one-sidedly, other arrangements are set up.
The proponents of gay marriage are not demanding equal status with marriage. They are seeking the cancellation of what many would see as the purpose of marriage: the procreation of children and the social affirmation of the status of women as equal partners in their creation and upbringing.