Maybe the English word “sex” is to the Japanese what the French word “soirée” is to English speakers—i.e., a foreign word that carries more meaning than its indigenous equivalent. Maybe “sex” sounds really dirty to Japanese ears. Maybe it’s just conveniently short. We don’t know the answer, but below are six posters using that magical English syllable. However you say it, just do it.
Call it whatever you want, as long as you make sure you're doing it.