1. Pastries. Cream horns and cannoli both have more or less the right shape, but we'll point out the obvious: they're little shells filled with cream. What happens when you insert them? The shell crumbles (or dissolves into mush, depending on how fresh your pastry is), the cream oozes, and the recipient ends up with a yeast infection.
2. Biscotti. Hopefully this doesn't need that much explanation--they're hard, they're abrasive, and they're made to absorb moisture. Why would you think that's a good idea?
3. Pickles. Okay, they're the right shape (assuming that you're buying whole pickles and not spears) but they have basically every characteristic that you'd hope to never see on a penis: bumpy, squidgy, and green.
4. Wine or beer. On the body, sure. In the body, and before long your characters will be too drunk to get it up. Their bodies would absorb the alcohol like they were drinking it--only faster.
5. Gelatine. We're not sure how you'd get it in, to start with, but then we're not sure that you want to watch it come out: gelatine will melt at body temperature. You put that in someone's body and you're in for a river of luridly colored melted gelatine to be pouring out soon. Plus, the dye in gelatine will dye your skin and mucous membranes just as well as it can dye your countertops and white tee-shirts, and no one wants a purple vagina.
6. Bananas, peel-on. We know, we know. They're the right shape and everything. On one end, though, you have little pointy nubs. On the other end, you have the part that attaches it to the bunch. Also, unless they're really underripe, they're squishy. You don't want to make bananasauce while you're having sex.
7. Cake or brownies. It's crumbly, it absorbs moisture, and let's face it: it's just better with ice cream.
8. Shortbread. See also: cake, biscotti.
9. Popsicles. It's one thing to drag them down someone's body, it's another thing entirely to put them in someone's cunt. At best numbing, at worst painful. Also, think of your sheets!
10. Salami. Or, really, any kind of meat. We won't insult you by explaining this one.
11. Hot dogs. They're soft and floppy anyhow, unless they're frozen, in which case they're disgusting and cold. If you insist on using them, please at least don't eat them afterward.
12. Icing, chocolate syrup, and honey: three great tastes that work better as something other than lube. On the body = tasty; in the body = ick.
13. Whipped cream. In the body, it's introducing air to the vagina, which can cause embolisms. Also, whipped cream foams, and who really wants a rabid cunt?