At least one of the major national 'sororities' is chartered nationally as a fraternity, although it's identified in college Greek systems as a sorority. A co-ed fraternity could remain a fraternity.
The issue here is that the national fraternities that charter fraternal chapters at Wesleyan may not permit female members. Wesleyan is trying to change the terms of agreements that the Wesleyan fraternity chapters have with their national fraternal organizations.
I thought it came from the Latin word fraternitas (brotherhood)--from frater (brother).