For the vast, vast majority of people, other adult people are the only appropriate sexual partner. Even furries look to other people, even though they may be wearing a fursuit at the time. By and large, people do not look to animals, coffeetables, or bales of hay to satisfy their sexual taste. A very few do, but they’re called something else – zoophiles, perhaps, or paedophiles.
There is, however, a much larger segment of the population that defines anything outside of the realm of one-man-one-woman as something dangerous and immoral, on the slippery slope to messing with infants and furniture and sheep. This definition of pansexuals, I don’t believe, is nearly as valid (from the pansexuals’ point of view) as the people who would prevent things like gay marriage would have you believe.
It also makes me wonder about the people who espouse the “slippery slope” hypothesis of sexual taste. Are they afraid that if they stray pat the bounds that their society finds acceptable, they will soon want sheep (and maybe secretly do already, and are only held in check by public or religious mores)? Well, I’ll say this—pansexuals do color outside of the lines, genderiffically speaking, but we’re proof that if you loosen up a bit, it doesn’t have to come to barnyard antics.