Seems like it's been much more than 3 days, but either way...They're in a zombie apocalypse. She's not dating anyone. She's human. She gets horny, like everyone else. The Governor seems like a very nice guy (although we, the viewers know he's not). The 'leader' type as was Shane, the only other guy she's slept with in the past year and a half or however long they've been out there.
Judging by that, Maggie's a whore as well. Her and Glenn only knew each other for what, a few days/weeks? Hell, how many times have Lori and Rick had sex since he came back? Oh, but they're married so it's okay. It would be a sin against the holy lord to have sex with someone you're not married to or in a relationship with.....unless you're a guy.