Google 'gaslighting.' People who'd never cheat don't typically have the need to vocalize it. Just like I never tell girls I meet, "Oh, don't worry, I'm not gonna rob & murder you. I'm super against people who do that sort of thing"--since that kind of thought wouldn't even be a part of my reality.You may take objection to this, but she told me when we first started dating where she stands on cheating and how she doesn't tolerate it. I don't think she would ever physically cheat on me. Tell me I'm wrong, but she seemed pretty sincere about that.
People of character never need to vocalize it. And people don't vocalize things they're not thinking of. By the same principle, when a girl say something like, "Ok, I'll come over, but we're not having sex," it means she's thinking of sex with you and it's much more likely to happen.