I disagree. The tag is there to warn users that NSFW content will be shown if they click it. Whether or not they decide to heed that warning is not our responsibility.
Forcing users to make an account to show saucy stuff isn't necessary. Our rules allow only up to softcore NSFW; we don't allow pornography, nudity or similar stuff. If we were a site where there was a constant risk of showing actual pornography behind NSFW spoilers, I would've agreed, but we aren't such a site.
Not to forget adding that requirement would only lower site accessibility. You and I both know not everyone is willing to create an account. If we block guests from viewing NSFW entries or images, we only lose visitors.
Now there's your argument about curious kids. For teenagers (still kids), whether or not they heed the warning is not our responsibility. The content that is allowed behind spoilers isn't 18+ following our site rules, so should be viewable for them.
Then that leaves only the actual small kids, but I've thought of that. Cheezburger's Terms of Service and Use state that you affirm you’re over the age of 13 when visiting the sites within the network. KYM being a part of Cheezburger, I think you can confirm we can use that as a guideline here. So basically, small kids shouldn't even be visiting this site. Our terms are listed, the responsibility is now with the parents and how well they control their kids when they're on the internet.
Edit: Dammit Verbose, beat me by 10 minutes.