See, that's what I thought when I saw it too. The asses just aren't quite shapely enough to be dead certain those are girls, and those are men's shirts.
The coloring could fit either pair as well. It's kind of brilliant if the vagueness is intentional. Which again leads me to ask, does anyone know what book this is from? It's page 43 of something, and not Die Sterne, Der Mond, or the Ecchi book for the Eva strip mahjong game. I must know!