>one would rather have them do team activities like sport or boy/girl/mixed scout

Plenty of developed countries where mixed team activities are the norm and the fertility rates aren't looking that much better once you filter first/second generation immigrants (including, many are already bordering on Japan's levels).

Having them do sports together until the age of 18 isn't going to solve the increasing cost of milestones, consumerism, increasing standards pushed by multimedia, crushing work cultures, pushing the norm to 30+, and more. Several of the aforementioned countries don't have half of these and still sit at 1.6 or less, while continuing to drop.