how can usui's 吻乐队（Kiss） on ayuzawa's back "while she was wearing a bikini" leave a mark ???

i watched the 日本动漫 and read the 日本漫画 for about 8 times in total but still cant understand,,,,i know i am stupid but just till me: how can usui's 吻乐队（Kiss） on ayuzawa's back "while she was wearing a bikini" leave a mark ???
日本动漫 : episode 16 -or 17-
日本漫画 : chapter 18 -or 19-