Perhaps the fraternities can rally the support of their alumni to accomplish what Taualumna suggested, and that is to house a fraternity & sorority in the same "building." The idea is it would be akin to co-ed dormitories segregated by floors, but perhaps separated by walls.
Since it seems a number of alumni have stopped giving or altered their giving, perhaps those missed dollars can be funneled into the housing corporation(s) of the fraternity (and sorority).
Even better would be to take two houses next to each other, and somehow building a cool tunnel or enclosed walkway between them to join them as "one living complex" haha
Seriously though, you can try forcing fraternities to house women (if they don't already or are opposed to it), but it's gonna work as well as trying to force a democracy on a certain middle eastern country.
Hybrid Mode
