I think that smoking could be a sin ... given its impact on health, vitality, etc. But only if it's a habit. There's nothing inherently sinful about, say, having a single cigarette, on social occasions ... or even a few here or there. For some years, I used to go out once a week and have on cigarette, after dinner ... which a group of us men enjoyed after singing Sunday Vespers at our church. But habitually smoking could be a sin if it impacts health. It's also expensive, so you might be impacting your family financially.
I doubt that there's a high risk of death with mountain biking. With skydiving, it was just reported on the news that one particular local skydiving location near me has had 20 people die since 1999, so one per year. As with anything, the risk needs to be balanced against its benefit. What benefit is there to skydiving that would justify the risk? Very little. If you want a rush, there are lots of amusement park rides that'll whip your around even in free-fall states.
Mountain-biking is not in the same category at all.
Now, I, as a husband and father, gave up riding motorcycles when I got married. There's a significantly-elevated risk of death vs. a car. You could be the best and most skilled rider in the world, but you're dead if some car-driver on the road doesn't see you. So what's the benefit, my personal amusement? That does not justify the elevated risk. Now, if I couldn't afford a car and the gas money for a car, and I lived in a temperate climate, I could potentially justify riding a motorcycle. But not just for my own amusement.
Topic: What are things you consider sinful that might not seem sinful? (Read 1222 times)