Tia's characteristic was already defined quite early by the author. If you lose to orcs too much, Tia would fall for orcs too. From the beginning, corruption is always a double-edged sword in this game. It's not just
more "liberal" to do and accept more naughty stuff. About Penny's situation that's just because we've kick out the other competitors like Lucius and Henry.
The following extract is taken from the walkthrough:
Watch her cut wood once per day, lose to the charging orc while Tia is at 4+ corruption. To revert: Every time you win against the orc this will reduce Tia's interest in them.
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Should you have accepted Lucius help during Henry's arrival he will offer Penny a special medicine to reduce her stress (see 13.). Should you have allowed this to continue, walk in on a Monady when Penny is bringing her vegetables to Lucius. If Penny has C15+ she will tell Lucius that she feels differently.
My suggestion is controlling any girl's corruption not exceeding 15. Though we'll lose some scenes we won't lose any girls.