Funny, I can't imagine being overweight as anything but irresponsible (mod specific health issues). And it is pretty much all on you.

One thing that can be very helpful is to schedule exercise with other people (e.g. spouse). It's easier to get going with a group, it's more fun, and there is a social cost to canceling. You don't even have to do the same exercises -- just go to the gym or pool or park or whatever and do your thing.

i.e. don't nag against bad activities, but support and encourage wholesome ones in their place.