I don’t think it’s all that clear that it isn’t sinful, at least according to a number of posts I’ve read here. I have struggled with this question myself. Prior postings on this topic have held that such thoughts even about one’s spouse are sinful because, in the absence of the person, they objectify her, which then becomes the definition of lust. Or, that such thoughts constitute a disordered level of erotic preoccupation. Or, that such thoughts incite lust because they can’t be acted upon at that time. There was even a post some time ago that referred to the “sin” of “anticipated intercourse,” whatever that is. I don’t necessarily hold these to be true, just re-stating opinions previously posted.
What do the Magisterial documents (as opposed to, say, Christopher West) say on this?