how can usui's 키스 on ayuzawa's back "while she was wearing a bikini" leave a mark ???

i watched the 아니메 and read the 망가 for about 8 times in total but still cant understand,,,,i know i am stupid but just till me: how can usui's 키스 on ayuzawa's back "while she was wearing a bikini" leave a mark ???
아니메 : episode 16 -or 17-
망가 : chapter 18 -or 19-