Nobody has said that here. You are probably confusing the holiday 'Jack in the Green' tradition (see Georgina's post) with the unrelated 'foliate head' carvings that have appeared in churches since the Norman period. Nobody called them 'Green Men' until Lady Raglan in 1939 (Folk-Lore 50, 45-47), but ever since then the romantics have been imagining all sorts of extravagant things involving pagan fertility rites and the like.