There are some words that should never be uttered, except ironically, by people over the age of 10. I'm putting "yum" on notice: You're going down, jerk. My loathing of "yum" was sparked by Rachael Ray's popularization of "yummo." If you Google "yummo," her Web site is the first hit, so she can't even hide from her dirty deed.
Your days are numbered, yum. There's nowhere to hide, for you or your variations, like "yummy" and the worst, "yummy in the tummy." Don't even get me started on "tummy."
I'll make one exception: If you are speaking to a small child and use "yum" and "tummy," I will let you live. Just don't make a habit of it.
This has rapidly become a multifront war. I'd better stop.