That's the effect of corruption. Maybe not universal. For example, if you keep Kate's corruption not exceeding 15, Kate will never agree to have 3thome with MC and Rick. I also hate to see my girls cheating on me voluntarily so I always control their corruption level carefully.
I know that corruption unlocks scenes. I think that's mostly the point, the girls become more "liberal" to do and accept more naughty stuff.
But you shouldn't be hesitant to raise corruption because it would mean NTR, it's a crap mechanic otherwise since the game pretty much requires you to do so (goodbye bonus scenes).
Corruption shouldn't make them actively want to cheat on the MC. Meaning, that it doesn't exclude the love relationship that they have with him.
Penny for instance is addicted to anal in my game, but she wouldn't shut up about it getting married to the MC.
NTR only happens when the MC himself puts them in those situations.
I don't see how that applies to Tia willing to cheat on the MC just because he's tired to have sex with her after everything they've been through. And then he has to go running to avoid her getting close to Dasan. So Tia is pretty much an untrustworthy liar.
But maybe I talked about this enough.