[ghaff]

There were a ton of membership organizations that, probably even quite a bit later, required one or more members to vouch for you and generally excluded people who didn't fit the generally WASPy mold. (And often male--male only organizations were quite widespread until relatively recently and AFAIK frats--as well as sororities--can be single-sex but mostly based on the fact that some level of living arrangements are OK to restrict one sex or the other.)

[ioblomov]

Surprisingly, Columbia didn’t accept women until 1983. Like Harvard and Radcliffe, it was affiliated with the all-female Barnard.

https://www.nytimes.com/1982/01/23/nyregion/columbia-plans-t...

[ghaff]

A lot of well-known private colleges/universities were single-sex, albeit sometimes with exchange programs/affiliates, until relatively recently and there remain significant single-sex, especially all-women, colleges today. Dartmouth, another Ivy, didn't go co-ed until 1972.

A lot of universities, especially those more focused on science and engineering, that were technically co-ed--even from their start in the 1800s--also didn't have a lot of women for a long time. The percentage of women at MIT was still under 20% in the late seventies.