I agree that generally speaking the warmer it is the lazier I am, but it's not quite like the colder it is the fewer ways of procrastinating I have.

In this grey lowlight world of English weather, with the added factor that by and large interaction between anglophones is mostly linguistic, it is not admirable that many come to office even on Saturdays and Sundays, and also stay late during weekdays. Basically, where else would they go?

Stay at home and watch TV/mess around on the Web/whatever? (EDIT: if you want something more old-fashioned, make that reading novels or knitting by the fireplace.) Go to the pub and play darts/pool with their friends?

Groups of young people still summon the strength to face the cold in particular for two activities: Training for sport competitions, and staying in line for a dancing club.

I think that's partly cultural. Where I am, it's not that uncommon for there to be plenty of young people just hanging around in the streets (going into bars to get drinks, but not staying inside each bar very long), even with below-freezing temperatures. To the extent that many such groups of people include both single males and single females, you could argue that on some level it's analogous to the dancing club thing, but still. (And snowfights are fun, though you might count them as sport.)