as are the Boy Scouts, Girl Guides, Methodists, and the Elks.
I believe that it is not the specific information here that is important, it is the demonstration that these women have a somewhat shared cultural (and class) background, having belonged to a sorority. This is the key. It shows that even though they may have just met, they have some background and life experiences in common. Not particularly important itself, but it certainly sets the stage for Jack and Randall, allowing them to immediately be more comfortable with each other and hence leading to the upcoming invitation of a little fishing trip get-together to get-away from their sorority wives. A subtle indication that they may have more in common than just the choice of wives, to me a brilliant construction by the writers to lead the viewer on to the next logical step that these men are "brothers" as well.
yes indeed, the invite to Lake Kemp [86 miles away, "down Mexico way"] was very clever of Ang in that Randell does not actually say "just we two" at all but everyone is "induced" to that, if not at the time but after OMT says "bring another fella up here".
it would be totally normal after first meeting a new couple in town to go away as 2 couples, blokes a fishin and a drinkin while the "little ladies" powder their noses inside talkin of THEIR priorities in life.
kinda like a little Hitchcock style "fishin trip" [npi], which then feeds into the SS saying "Ennis now knew it was the tire iron"
as for
"as are the Boy Scouts, Girl Guides, Methodists, and the Elks"
yes my point entirely, ie this all happened just before Homosexual Affirmative Action circa 1980 but was DEAD IN MIDDLE of Women's Affirmative Action circa 1970 [aka Women's Lib], and IMHO Ang spends half of the movie showing us 6 different types of women [7 if you include Mrs Agirre] in their own personal "relationship" with such monumental changes to society - hence my far greater interest in that aspect than the simple issues of we mere males of the species.