Aww, at least she came clean and apologized to him now. She just wanted to hang out with him and he promised her that they would.
As for this rumor about Luna being a slut, I am thinking that is the reason she never went back. The more that goes on the more convinced I become that she really wasn't, but someone threw a rumor out there that she was a slut and it probably hurt her (even if she didn't show it). It could be the guy that she kissed that Nao saw. She told him she wasn't interested in him and dating and he got mad and started spreading a rumor that she was a slut to make himself feel better OR some girls (specifically one that liked this guy) found out about the kiss and started spreading a rumor about Luna being a slut to get back at her.