Or Micha simply has some actual feelings for her ex and would rather be with him, given a chance. This doesn't need to have anything to do with Rusty's reputation, or lack thereof.You should read the whole scene again. Of course, Micha rejects Rusty and prefers to try again with her ex, because Rusty has lost his reputation. He has screwed around a lot and is a male slut. As a result, no decent girl trusts him anymore.
Furthermore, i doubt Rusty really has any reputation of such kind, he's not exactly shown sleeping around compared to other DIKs. The only thing we know about him is what Jill has told the MC, that he'd put up a fake personality in front of his (presumably DIK) male friends, and that's what turned her off.