Long answer will take a bit more. First of all, all you speak about is sexual activity. Zoophilia is about far more than just sex. Zoophilia is when a human loves and respects nonhuman animals as true equals and is emotionally/romantically attracted to one or more species as mates. The human may or may not also feel physical/sexual attraction. When the zoophile is also physically/sexually attracted to those animal mates, that is called zoosexuality. Zoophilia/zoosexuality is an inborn sexual orientation no less than heterosexual, homosexual, bisexual, etc. A person who has only a sexual interest in animals with no emotional attraction is called a bestialist, because all the person is doing is bestiality. Bestiality by definition is sexual activity between human and nonhuman animals. While zoosexual sex is technically bestiality by definition, there is a clear difference between a zoosexual person and a bestialist due to their attractions and motivations.
Zoophiles/zoosexuals always pay very close attention to their animal mates to determine consent and rejection to various activities, including sexual activity if that's something they do, and respect and obey that decision. Bestialists may or may not pay attention to that consent or nonconsent; some still respect animals to obey a rejection and move on, while others will ignore that lack of consent and proceed with sex anyway, using any method of coercion, threat, punishment, restraint, or sedation. Rape does happen, and rape is rape regardless of the gender and species of the victim.
Animals are fully capable of forming decisions of consent/acceptance or nonconsent/rejection in all things including sexual activity with suitors and mates, and are fully capable of communicating that consent or rejection through nonverbal communication of vocalization, body language, self defense, and attack. Such behaviors are easily seen in any nature show which shows the obligatory "animals having sex" scene, where suitors get rejected while others get accepted. Male animals are not going to get an erection if they are not in the mood or otherwise don't like what the human is doing. Likewise, if they are in the mood, do want some sexual attention from the human, or do enjoy the sexual pleasures being given, they are going to get and erection and continue participating. Just the same with females, if they do not want sexual attention they are not going to present their genitals or permit continued contact, while if they are in the mood or enjoy what pleasures are given, they will maintain a sexual stance with their tail off to the side. In either gender, if they really don't like what's going on they are going to move away, sit or lay down, turn around to face the persistent person (of whichever species), growl, flatten ears, and eventually use teeth, claws, or hooves to get the point across.
Two cases in point: when my female dog is in heat she'll present herself to my male dog and do all she can to get him worked up. He's a nutless wonder so it takes him a while to figure out what to do. Eventually he figures it out and accepts her invitation for a wild ride (and soon forgets what he was doing and falls off of her). When she's not in the mood such as when she's trying to sleep, she will sit or lay down if he tries humping her. When she's towards the end of her heat and hormones are conflicting with lack of desire, she growls quite aggressively when the male tries humping, and will walk or run away until she gets a chance to sit down and thwart his advances. In the other case, a someone I know has a female horse who is incredibly horny when she's in heat. She'll spread her legs and squirt (it's what mares in heat do) when her name is simply called. Scritch her butt by her tail and she stands ready to be mounted with her tail off to the side, winking her vulva (it's what mares in heat do). When she's not in heat, a scritch to her butt by her tail will result in a kick backwards hard enough to nearly break boards in her stall. If someone can't understand those messages of "yes, take me now" and "NO!", then that human deserves to get hurt.
So long answer summarized, if the animal is fully consenting to sexual activity with the human (and of course as long as the human is fully consenting to it too), then there's nothing wrong with it. It's just two sexually healthy adult creatures sharing intense physical pleasure together, and whether it's because they're in love together or just want sex, there's no harm in that.
To reply to the other answers, "Paul Jackson" claims animals can't consent, which is very much false. He claims there is a public health problem with the potential to spread disease. While that is true if the animal is not up to date on his/her healthcare and parasite control, since some bacterial infections and a tiny number of viruses are zoonotic (can cross the species barrier) and parasites often don't care what the species of the host is, sex with a healthy animal would actually be a lower health risk than sex with another human because ALL human diseases, sexually transmitted or otherwise, can be spread between humans. The claim of it being unsanitary is eliminated by simply bathing the animal partner, and the statement of it being "aesthetically unpleasing" is merely a statement of personal opinion. Everyone is entitled to their opinion but it does not constitute a logical argument.
"Anonymous" is correct that a lack of resistance does not necessarily mean consent because an animal trained to obey through disciplinary means is going to fear that discipline if he/she doesn't allow the human to do whatever. Consent requires the animal to know he/she has the freedom to express displeasure with something the human is doing without being punished for it. Sometimes that displeasure must be violated, as in things like tooth brushing and veterinary care, but the rest of the time if the animal has that freedom to object then he/she has the freedom to consent. The "alpha male/female" mentality is also a matter of training and environment; if the human treats the animal as an equal, the animal knows the human is not the absolute alpha and he/she doesn't have to let the human do just anything. Erections are not as instinctive as "Anonymous" thinks. It does require a certain level of desire, and forced sex has no desire due to the victim wanting it to stop. If an animal doesn't get pleasure from sexual contact with his penis, the human is not going to be able to make him get erect for sexual activity. On the other hand, an animal can proposition the human with a very clear communication by showing an erection and humping or presenting a ready vagina. In that sort of instance the desire and consent should be unmistakable. "Anon" claims there is no evidence the animal enjoys copulating with the human. The facts are sex feels good, animals like sexual pleasure, and so many porn videos showing a human with a very enthusiastic animal clearly show enjoyment on the part of the animal. This is basic observation and rational analysis. His/her statement of it not being moral or legal is irrelevant. Morals by definition have to do with what's truly right and truly wrong, whereas the common social usage as this person is doing is using "morals" as a statement of personal opinion and agreement. Legality is irrelevant because something being illegal due to social objection doesn't make it wrong, for example interracial marriage, homosexual activities, or even heterosexual non-vaginal-intercourse activities such as oral and anal sex that used to be very illegal. Clearly none of those things are wrong and such laws have been struck down. And no, the same logic cannot be applied between human children and adult animals. And in his/her update, while it's true that some human rape victims do experience orgasm as an involuntary response to the intercourse of a rape, it's a fallacy to imply that all sex (with any species) is automatically rape regardless of pleasure or an orgasm.
The lack of resistance does not necessarily mean consent. Some pack animals are subservient to their "alpha male" or "alpha female", and will not protect themselves if attacked. In fact, these animals would just bare their necks, indicating their submission. This does not mean the animal welcomes the attack.
Furthermore, erections are instinctive. The presence of an erection or even an orgasm does not indicate consent, because those are not necessarily linked to the fore-brain.
Because neither of the characteristics I listed previously necessarily indicate consent or enjoyment, I claim that the animal(s) you describe do not necessarily "want" to copulate with a human, or consent to such activities. You also have no evidence that the animal enjoys the act of copulating with a human. Until proven otherwise, the animal must be assumed to be a non-consenting party.
Finally, consent does not necessarily imply moral or legal correctness. Children can agree to perform sexual activities with other humans, but this would still be statuary rape. This is because we assume that they do not realize the full import of their actions, and so their consent is non-significant morally or legally. The same logic applies to animals.
Update:
Question: "If an erection is a natural instinct then why dont they resist?"
My response: Instincts are by definition hard to resist. If you're sleepy, it's difficult not to yawn. Similarly, erections can be caused by physical stimulation that the body cannot resist. It's like jerking your knee when the doctor hits it with a mallet: you can't. There are documented cases of humans experiencing erections or orgasms during rape. That does not mean it was consenting, or that the person did not resist. I do not see why this does not apply to animals as well.
Update 2:
User "Sky" makes many very good points which I am unable to refute. It is clear I will need to do further research here.
Animals can't consent. The public health problem with bestiality is that there is a potential to spread disease between species. I see the practice as extremely unsanitary and aesthetically unpleasing. I can't understand how anybody could bring themselves to do it.
the real question is this: WHY THE **** DO YOU WANT TO BONE AN ANIMAL?
You may say that we cannot know for sure what the animal says it wants but you can see and hear by certain body languages and sounds like if it has an erection, its obvious it wants to have sex with a human so the consent part is clearly there.
What do you think is it right or wrong if the animal consents and why?
