Do you guys realize what this means? It means Makoto was asking Haru to bring him a towel because he was naked. And judging from his body angle and how far open the door is, Haru would obviously see him naked. And Makoto was completely okay with that. Meaning Haru has seen Makoto naked before.
…Or, you know, nudity in Japan isn’t really a big thing, especially when it is between two people of the same-sex. Community bath houses are really common and friends will go all the time, so you wash up and relax in a bath completely nude with your friends. Nudity and sex are not as big a deal in Japan as in many other parts of the world. You can literally walk into a Lawson’s (convenience store) and right in the back there will be a whole shelf just full of porn (so boobs and butts hanging out everywhere…one time at a 7/11, when I was still a college student, my friends and I found a porn mag for women, which was rare!). So, yeah, Haru’s probably seen Makoto naked a lot of times. He’s probably seen Rin and Nagisa and Rei naked too. They are friends and would be comfortable around each other.