Here is a portion of the notes I made when I recently took a course on the history of women --
“Gender” refers to the social roles for men and women. These roles are constructed by society and are influenced by race, politics, religion, class, etc. However, it is a dialectic, with society affecting gender roles, and gender roles affecting society, which is how gender roles change from era to era. This is true because there is often a conflict between social theory/ideology and social practice. The roles for men and women are always relational between men and women. They are also part of a power relationship.
Biology assigns us sex roles, but society assigns us gender roles. Nowadays, gender roles are changing -- men becoming househusbands who have the primary role for rearing children; women becoming soldiers.
It's been a while since I read the article, but I think his point was that for most people, marriage is not a matter of gender roles but of sex roles; most people define marriage as between a man and a woman, regardless of changes in gender roles.
I also think he's implying that it's harder to change society's idea of sex roles than it is to change society's idea of gender roles.