An indoor trainer will not cause damage to a bike, the forces are minor compared to riding on the road.
I ride 5 days/wk, 1-2hrs/day all winter (jan-march) on Spinervals DVDs -that's 53X12 sprints and 155 rpm spinning in easy gears. It's hardly boring, unless you just spin around 100rpm straight in an easy gear watching TV, which is common, and useless.
Local spinning classes are great for housewives, but unless you have a high-level group, they are weak sauce for road riders. Plus, you never get proper bike fit and spinning is about weight loss, not cycling performance.
Three tips: 1. buy a large fan, 2. use a trainer-specific tire. Continental or Tacx make them, ~$40. 3. Buy training DVDs (Spinervals, Carmichael), they are nice because they are calibrated and they will boost fitness -allow you to compare to riders at many levels, but they are HARD. Some are set to real rides outdoors. Also, Kurt's are calibrated with power to speed, so you can calculate power output -download the charts from Kurt website.
If you are really using a trainer properly, you will shred a road tire in about an hour. Road tires are not designed for the heat of the roller. Trainer tires last a long time.