Is that because most marriages exist in large urban centres where cooperatives are uncommon to non-existent? Out here in rural country, where cooperatives are the norm, most married people invest in cooperatives as most of the cooperatives are customer owned. You don't have much choice but to invest, assuming you want to be a customer – and no doubt you want to be, given that they provide basic services for the region.
I'm certainly an investor in several co-ops, and not because I went out of my way to do so. That is, with the exception of one co-op, which I provided additional investment to. That particular co-op runs a very successful operation and provides healthy returns in what is a comparatively stable environment to those who provide an over-and-above investment. They attract quite a lot of investment dollars as a result.
Which says something about unions that aren't investing. Why aren't they investing? Simple: The companies they see in front of them aren't worth investing in.
Neither does the Union of European Football Associations, otherwise known as UEFA. They make up for it with a dope anthem.
Neither the Kalmar Union, nor any other personal union of monarchs, ever invested in a cooperative. Although some may have invested in joint stock companies, such as the various East India companies.
Maybe the Soviet Union was into worker cooperatives? Not sure.
And finally, the union of the set of workers at cooperatives and the empty set invests in cooperatives. Hey look, we found one type of union that definitely invests in cooperatives!
(I was just having fun making this as absurd as possible. Sorry, no malice intended).