Two points strike me on this topic. Firstly, any decline can't have anything to do with the all male-ness of some sides. Do football cricket and rugby have a problem? No? Do they have all maleteams? Yes.
Secondly, the quote about incest refers to country dancing, not morris dancing. And, specifically, about the kind of country dancing purveyed by primary school teachers between the wars. And who could deny its validity?