7. A frat boy and a crab
This actually applies to men, women, and any STD, but this was the most likely scenario we could come up with. Sure, it'll be funny to some of the (presumably drunk) people around you, but do you really want to publicly represent an STD (and/or someone who has an STD) all night? We wouldn't. It just seems like you'd be testing the disease-ridden fates with that one.
6. Jamie & Cersei Lannister
You know what's an awesome show? Game of Thrones. You know what's not awesome about the show? Incestuous rape. Even if you're going by the book version where there is no rape, do you and your significant other really want to spend the night pretending to be siblings/lovers? If you've got your heart set on a Game of Thrones-themed costume, what about Jon Snow and Ygritte? At least then you'd get to go around telling people that they know nothing. For that matter, we'd dress as pretty much any GoT couple before we'd go with the two full-sized Lannister siblings.