Because the God speaks of them as if they were horses that wanted to mate with other horses. Alternate translation: "They were like horses in heat ... neighed to his neighbor's wife" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Male horses neigh loudly to female horses when they want to mate with them. Alternate translation: "Like horses who mate with more than one horse, these men wanted to sleep with other men's wives" (See: [[rc://en/ta/man/jit/figs-metaphor]])