I will guess.The number 5 is the smallest unit of perfection as it forms the basis for the larger numbers of perfections such as 100, 1000, 100,000, etc.  As a Greek Pythagoras would have known that these significant numbers would not be divisible by any other meaningful number smaller than 5 (the number "1" notwithstanding).  The relationship to marriage is that marriage provides the smallest unit of relationship within each community.  As a small but significant institution, marriage would provide the "bedrock" or larger society.  Society could in a sense be considered as a collection of "marriages" and the families they provide.

Nope  ;D

Darn.  You have to admit, though, that my answer makes sense in its own right.  🙂

Was it because the typically or ideal Greek family was comprised of two parents and three children (five in total)?

5 is the sum of 2 (the dyad… 1st female number) and 3 (the triad… 1st male number).source: http://www.mathgym.com.au/history/pythagoras/pythnum.htmCheers,Waly

And we were supposed to know that off the top of our heads???  Here I was thinking it was something more evident….I guess I never knew that there were guy numbers and girl numbers.  ;D