They certainly fuck if Penny's a cumslut and not hiding it. As for the 'mate' thing: when it comes to this game's NPCs, it's generally best to assume polyamory (or at least open relationship) until given significant evidence otherwise - terms like "mate" and "[pc.boyGirl]friend" aren't good enough for that - and Penny (especially when she's a bimbo or cumslut) is no exception.