In a time of equal opportunity, it is unfair for any university to exclude anyone based on gender, sexual preference or race. Although it is historically an all-female college, times have changed. Male colleges have been forced to accept female students. Smith is no different than any other college that needs to provide equal education opportunities.
I don't think that there's any reason for Smith to become a co-ed college. I think that in this day and age, we ought to allow women only and men only colleges as a sort of niche experience for student consumers who want that sort of educational environment. There's no need to diversify everything.
Smith College in Massachusetts is a private liberal arts college. As such, it can allow and disallow whatever students it wants to within reason. If Smith College wants to stay all-female, so be it. If the institution ever accepts taxpayer funds to help run its programs, then Smith College must change an accept everyone regardless of gender.
There is no reason that Smith should become co-ed. As long as there is a market for an all-girls school there will be all-girls schools. The day this demand ceases will be the day the single gender schools cease to remain profitable and self-sustaining. The market and the demands of the market are more than adequate to regulate the services and goods the market offers. Smith should become co-ed only when it is profitable to do so.
Smith College has historically been and was created as a college for women. It is also a private institution that has every right to serve what ever part of the population they so desire. I do not believe the school should switch to co-ed unless the directors desire to do so.