They don't "know" about your porn bookmarks, it's just stored on their servers, besides the sync is not obligatory you can use the xmarks extention if you're more comfortable with them.
I know, I can use xmarks or del.icio.us or whomever, but having bookmarks/extensions or (at least eventually) all of my browser state synced in a single place... that's just unbeatable IMO. Especially because I have a desktop, laptop and work laptop that are used about equally. It's a nightmare to remember, "Oh where did I bookmark that page at?" etc. Not to mention extensions, cookies, etc, etc.
Paranoia or convenience. Take your pick. You can't really have both.
If you use your machine, they never see it, and if you use theirs, they can't decrypt it.
Chrome's syncing? Not so much.