I think a friendly bet on a footbal game or some other event (not necessarily a sporting event but other events also) can be fun sometimes, if it doesn't get out of hand. As with everything else, it can be taken to extremes also if one doesn't watch what they are doing.
Still, it can be fun and adds a little bit of enjoyment to the event. The "bet" doesn't even have to be for money, as it can be for "time spent together", "doing chores", and stuff like that. And, if the stakes aren't too high, then you don't lose much.