Incest and intergenerational affairs are also taboo and more likely to remain so than bestiality. Zoophilia is legal in many states so long as it occurs where no one can see it. Not so the other pairings.
The Sex Positive agenda seeks to end all moral judgements against ALL sexual pairings regardless of sex, age, relation, marital status, number, or species of partner, Santorum was correct about defining deviancy downward.
I happened to think of a few more fiction examples that edge into the Zoo realm. In the 1980's "V" the miniseries, a lizard alien (appearing as a human) impregnates a human female. Star Trek has featured cross-species relationships and resulting children starting with Mr. Spock.