Preheat an oven to 350°F. Line an 8.5 x 4.5" metal loaf pan with parchment paper, leaving overhang (see photo above), and lightly coat with nonstick spray. Whisk flour, baking powder, and sea salt in a medium bowl, and set aside.
Add sugar and citrus zest to the bowl of a stand mixer fitted with the paddle attachment. Using your fingers, work the zest evenly into the sugar. Add eggs, oil and vanilla, and beat on high until light and thick (3 - 4 minutes). Reduce speed to low, and mix in half of the dry ingredients followed by 3/4 yogurt. Mix in the remaining dry ingredients, then 3 tablespoons grapefruit juice, 1 tablespoon orange juice, 1 tablespoon lime juice and poppy seeds. Scrape the batter into the prepared pan, and smooth out the top if needed.
Bake cake until golden brown and a skewer inserted into the center comes out clean (50 - 60 minutes). Transfer pan to a wire rack, then poke holes all over the top of the cake with a skewer. Brush remaining grapefruit, orange and lime juice (will be 3 tablespoons) over the top of the cake. Let sit for 15 minutes, then run a knife around the sides to release the cake. Using the parchment handles, lift the cake out of the pan and onto the rack. Remove parchment and let cool completely.
To make the glaze, add powdered sugar, 1 tablespoon yogurt, 1 teaspoon water and a small pinch of salt to a medium bowl, whisking until smooth. Drizzle glaze evenly over the cake, then sprinkle with poppy seeds. Let glaze set for 30 minutes before serving.