You can't really keep them in iPhoto and have them protected. I'd say move your porn into a folder, and use disk utility to encrypt it. If you still want your porn to show up in iPhoto, you can change where iPhoto looks using iPhoto's preferences.
This little app will allow you to have multiple iPhoto libraries.
Keep your regular library, and then simply just create a whole new one for all the good pics....
Just remember if you launch iPhoto directly it will open the last library you view via the iPhoto buddy.
consider some or all of these:
1. Create an encrypted disk image (big enough to hold all your... ummm... "private" pictures. Make it big enough to account for future expansion). choose a password, and don't have it saved in your keychain.
2. open the encrypted folder.
3. when opening iPhoto, hold down the option key
4. create a new iPhoto lilbrary inside your encrypted disk image
5. use this new library for your.... ummm... "private" pictures
6. when you are down remember to unmount the encrypted disk image
Depending on your needs, you could do all or some of these (just put the pictures in an encrypted disk image, not in iPhoto), or just use a 2nd iPhoto library