[ealgeone]

Sinful curiosity is also at the root of a lot of lust and immodesty. A man may be happily married, but when he sees a woman walk past on the sidewalk he may temporarily push that to the back of his mind. Part of his problem is lust. And in that lustful mindset, he reduces the woman—a person—to her curves and other physical attributes. But another aspect of his struggle is the sinfully curious question “I wonder what she’d be like?”

What the Msgr leaves out is at this point the man has committed adultery. He's as guilty with this thought as he would be if he were to engage in the physical act itself.

[olepap]

...at this point the man has committed adultery. He's as guilty with this thought as he would be if he were to engage in the physical act itself

That's taking it way too far. Adultery is the physical act of sex with a woman not your wife. We all have sexual thoughts, this is designed in by God.

[Campion]

Not quite true. It's the same sin, but there's a difference in the gravity of the objective guilt between thinking about the act and actually committing it. There are gradations of "thinking about it," as well -- between a slightly willed momentary toying with the idea, and a decisive fixation of the will "I'm going to do this ..." -- which matter, too.

Somebody toys briefly with the idea of stealing a pencil, while somebody else commits armed robbery of a bank and gets away with $200,000 in cash. Both of them are committing the sin of "stealing," but it's ridiculous to suggest that the objective guilt of both acts is the same.

Otherwise you end up with morally ridiculous situations, like equating what Adolph Eichmann did with me getting angry at someone who cuts me off in traffic. Both are sins against the commandment "Thou shalt not kill".