To me, smut definitely has a much less innocent connotation.
Than just saying "sex"? I mean, I understand that "smut" has a darker connotation, but if there's detail involved and you're not airbrushing the act, it doesn't matter if you call it sex, naughty or smut. <shrug> It's nothing I'd want my 12 year old son reading.
I'd like to think that when I use those elements in a story it actually adds to the plot/characterization, but I also know that viewpoint is entirely subjective.