Hiking

I'm coming at this from the perspective of someone who has primarily hiked in the northeastern US, but I expect a lot of other places can also have surprise rainstorms or sudden temperature drops in the middle of summer.

Jeans don't handle wet well. They aren't very good at insulating while wet, dry slowly, and will be very heavy while wet. These problems aren't unique to jeans, but jeans are much more popular than other garments with the same issues.

It's not unreasonable to turn away folks who show up in jeans. Especially if they gave advance notice.

Even if it were unreasonable, it's their club. You can find others to hike with if you'd like to wear jeans.