The pilot of the glider took off under aerotow for a flight in the local soaring area. The pilot released from the tow airplane at an altitude of 2,500 ft and 3 miles downwind of the airport. While soaring, the pilot found a thermal and regained some lost altitude to 2,300 feet msl. The pilot then reported making several 360° turns in the thermal to gain additional altitude, but he then noticed that he was now 8 miles from the airport and decided to turn back toward the airport. While soaring back to the airport, the pilot was unable to find any thermals to regain altitude. When the pilot was 3 miles from the airport at an altitude of 1,000 ft, he determined he did not have sufficient altitude to make it back to the airport and began to look for an off-airport landing area. The pilot found a pasture that he believed would be a suitable landing site. During the approach, the pilot noticed there was a wire fence across the pasture. As he approached the fence during landing, he retracted the spoilers and pitched the nose up in an attempt to clear the fence. Subsequently, the stall warning activated, and the glider impacted a fence pole which resulted in substantial damage to both wings. The pilot reported there were no preaccident mechanical failures or malfunctions with the glider that would have precluded normal operation. He further stated that the accident could have been prevented if he had requested that he be towed and released upwind of the airport.