He doesn't wear pants under his yukata! That's because he wants to show off his erect penis to the masseuse. To do that, he needs to be able to remove his genitals quickly, so pants get in the way! He just wants to make it look like an accidental slip. That's the key. If he can do this naturally, the masseuse will get turned on the moment he sees his penis! No, he will get turned on!