Because thought implies judgement, and because we are all paranoid enough to take judgement to be negative, it is constitutionally suspect in the bedroom.
I wasn't thinking anything cruel while I ran my hands and lips across Chloe's body, it was simply that Chloe would probably have been disturbed by news that I was thinking at all.
Thought's infidelity lies in its privacy. 'If there is something that you cannot say to me,' asks the lover, 'things that you must think alone, then can you really be trusted?'