Yeah, Rick Perry tried claiming abstinence is infallible as well. Everybody knows that hormones trump brains far too often for that to be true.
Lust (thinking about sex) outside of marriage or if you're married and you think about someone else it's a sin according to the Bible, you don't even have to perform any kind of action.
Matthew 5:28 NIV - But I tell you that anyone who looks at - Bible Gateway
28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.