Decades ago, Kate Kain arrived at Toby’s Feed Barn unsure how many people would show up for that night’s gay dance. But word had spread, and about 100 people trickled in, bundled in jackets to counter the cold that would melt away beneath a disco ball. “I remember, at the end, standing in a circle and having a moment of silence because it was so wonderful to be at a gay dance,” Ms. Kain recalled.…