Ooh, a thorny one! To my mind, there are two different things that are being discussed here - what's 'right', and what's 'socially acceptable'. Currently most cultures consider any sexual contact with animals to be unacceptable, but historically that may have a lot to do with the fact that it's 'deviant' behaviour - i.e. will not result in offspring. Looking at common cultural and religious attitudes to homosexuality, paedophilia, necrophilia, prostitution, etc. they've almost always been considered distasteful at the very least.
Morally speaking, it's really difficult to say whether bestiality could ever be 'not wrong'. Although one could argue that a woman having intercourse with a male animal has implied consent, most of the animals we have day-to-day contact with are domesticated because they're trainable. Even if an animal initiates sexual contact, you cannot definitely say whether it's doing it because it wants to, or because it thinks it ought to. In human terms we consider 'unable to give clear consent' as 'non-consensual' - I don't think anyone would argue with that definition when we're talking about date-rape.
On the flip side of that, the point has been made a couple of times that we do a whole load of other things to animals that are morally questionable. Basically any domestic animal you see today would not exist without humankind 'messing with it'. Not just from the perspective of that individual creature existing (the world's commonest bird is the domestic chicken), but also that whole breed existing - the creature has been created to either work or provide food for us; in the wild it would never exist.
Without wanting to get into a tangential argument over modern farming methods; yes, intensive farming can be seen as morally objectionable, and many people prefer to buy organic products for that reason. Some people choose not to eat meat at all (ethical vegetarianism has actually been around for at least 2,500 years in the form of Buddhism), though a simple examination of your own teeth will tell you that humans are 'supposed' to be omnivorous.
Interestingly, having just skimmed the Wikipedia article on zoophilia, it's actually not specifically a crime in a number of countries (and US states). In the UK it is illegal, unless you kill the animal first of course. Unless you photograph or film it, in which case it's illegal again (not sure how much of a dead animal counts, but assuming it's any amount then would these - http://www.lovehoney.co.uk/product.cfm?p=8551 - be potentially illegal?).
Attitudes to particular aspects of sexuality do change over time; however they don't always become more liberal. Over the last 50 years Western society has become much more tolerant of homosexuality, BDSM, sex toys, etc., but seems to have become virtually hysterical over paedophilia (in the UK, anyway). Although I don't think that attitudes towards sex with animals are likely to change much, it's interesting to note that the Kinsey Reports (in the USA, late 40s and early 50s) gave figures for those who had experienced sexual interaction with animals as 8% for males, and 3.6% for females (rising to 40-50% in those living in farming areas). Even allowing for the fact that the percentage of the population in farming areas is much lower now, that's still a lot of people.
With regard to Pixieking's point about genital contact with animals, whether it's a vet with his arm up a cow, or B-list celebrities wanking off pigs, the difference I think is intent. In both cases the intent is not the sexual pleasure of either participant - it's either for the animal's health (or it's offspring's health), or to intervene in the reproductive process.
If I can also refer the earlier discussion regarding animals having sex for pleasure, I'd argue that a great number of animals (if not all of them) have sex for pleasure - they do it 'cos it feels good. It's fulfilling instinctive behaviour, and nature generally rewards that with feelings of pleasure. As far as I know no animals have the cognitive capacity to associate sex with future reproduction - they're just doing it to 'scratch that itch'. Bonobos and dolphins are unusual because they still do it for non-reproductive purposes - they're still scratching the same itch, but they've just evolved to use sex as a social bonding tool. Look at homosexuality amongst animals - it exists in way more than two species, despite the fact that there's absolutely no chance of reproduction.
Although I have to say I'm probably heavily conditioned by social taboos, from a purely philosophical point of view I have to conclude that if it's morally acceptable to imprison, mutilate and use negative reinforcement techniques to condition an animal to behave unnaturally (i.e. get a dog, keep it in the house, get it neutered and train it), then sexual contact is, from the animal's perspective, no worse. Of course, causing distress or injury to an animal will always be morally wrong, but if there's no physical or psychological suffering from the animal's perspective, who can really say it's wrong?
And finally, I've never been given 'the look' by a dog, but we looked after a cat once that came into season while she was with us - that freaked me right out.