Group belonging is a fundamental part of the human condition. We are social, tribal animals. You may, of course, choose not to belong in groups that are defined

Group belonging is a fundamental part of the human condition. We are social, tribal animals. You may, of course, choose not to belong in groups that are defined by being male, but people are members of lots of groups with boundaries drawn for lots of reasons. Given that many men have similar experience by virtue of being men, it seems a reasonable attribute to base a group on.