My hypothesis goes something like this:

• Sin means "off the mark" (not immoral). Meaning: not being perfect.
• Christian philosophers tried to figure out the mind of God.
• Natural Law was discovered.
• They deduced that if something was natural, it was good. E.G. Having babies is natural so all good sex should lead to reproduction. (or something)
• However, Natural Law ethics invoke the Naturalistic Fallacy (what is, should be).

Some folks are simply ignorant of observational cues. Just as we have our own methods of consent within our species, so too do other species within the animal kingdom. Obviously, the animal kingdom isn't exactly rife with rape, animals consent to each other. The animals with the most desirable traits get more consent with partners, this is the essential tick of evolution. That said, if you understand what cues to look for, you may find that your prospective animal partner has just the same thing in mind as you do. However, if you don't receive any cues, it'd be wise not to force yourself on an animal, that's the fast road to getting hurt and/or cause psychological trauma on said animal.

Anti-zoos always bring up the consent arguement, it's like their trump card. They just use this to legitimize their emotional reaction for which they have no sound argument to defend. Animals can consent, it is just via non-verbal cues. Besides, we do things to animals all the time that they would never consent to. You think chickens consent to being locked in cages their entire life? Or that cows consent to be slaughtered? But as soon as you bring the ick-factor of zoophilia into play, it's "They can't consent!" It's up to you to decide whether you believe their argument or not. Do you think they can consent?

Yes, it's true animals cannot verbally consent--many non-zoos use this as the gold standard of ability to consent, but of course animals will fail an artificial standard like this. After all, they're not human. They'll also bring up that there's no way to know for certain animals are enjoying things--which is true, we can't know with absolute 100% certainty. But given the similarities in mammalian brain organization (limbic system, etc.), and the body language animal exhibit which we routinely accept and intrinsically understand in any other situation, I don't feel concerned about that miniscule chance.

However, as others have mentioned here, animals are very capable of consenting in their own way. While it's ideal for you to know an animal closely before taking things to that level, so you can better read their behavioral cues, there are times where it's perfectly obvious. I've been around a female dog who'd simply let out a low warning growl if her comfort level was being breached, which I respected. I've likewise been around unbelievably enthusiastic and persistent dogs, to the point they've literally almost forced themselves on me! (not that I minded, mind you...)

Some more persistent folks will get into the idea of "informed consent"--that since an animal doesn't and can't know it's "being taken advantage of", it's therefore being taken advantage of. Do animals understand the biology behind sex, and carry around with them the emotional baggage we humans do in regard to sex? Probably not. But it's something they can and do enjoy in some situations, and they seem to be pretty good at deciding for themselves if, and when, they want to partake in it.

Lastly, there's the notion that humans are in a position of power over animals, and could do things like withhold food or affection if an animal's not "putting out". But ultimately, there's the same grounding of trust our society has: that people can keep animals, and won't mistreat them. Sadly, some people are pieces of shit and abuse their animals, and some so-called zoos are pieces of shit and sexually abuse their animals. But almost nobody is demanding that society abandon pet ownership because of the people in that former group; it's simply the squick factor that makes people bring up the argument in the latter case.

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The whole consent issue is moot. These same people who say an animal can't consent to sex, see no problem killing the same animal. They reason animals have always been used for food and clothing and that is "normal", but having sex with them is not, so is wrong.

Reality check. Animals can and do consent to sex with humans. Once you understand them well enough you can see for yourself.

And BTW, here's a couple old threads on the very issue:

http://www.reddit.com/r/SpaceClop/comments/xqihl/so_yeah/

http://www.reddit.com/r/SpaceClop/comments/xuqdz/a_serious_question/

A couple of the top comments on those threads:

http://www.reddit.com/r/SpaceClop/comments/xuqdz/a_serious_question/c5refqw

http://www.reddit.com/r/SpaceClop/comments/xuqdz/a_serious_question/c5q892o

WTF? I just checked the links and see that /r/spaceclop has gone private? Or is undergoing repair??? Fuck. I hope they are not erasing old posts.

Consent is a made-up justification. In earlier times they would have told you that it is against God's will. Nowadays consent vs rape is en vogue.

These reasons are interchangeable like plug and play, because they are justifications after the fact. People first get disgusted or whatever, and then make up a "logical" argument to be able to get rid of it.

Consent: Either they will argue animals can't talk. That is stupid, there are many threads on reddit alone about how well owners understand their pets. Whether they are happy, what they like, if they need a walk etc etc. But all of a sudden for sex in particular such communication, e.g. by body language, by mooing, neighing, growling or whatever is impossible? Bullshit. Moreover, the same people are frighteningly unable to produce documentation that their last human partners consented to the sexual intercourse. So much for that.

Or they will say the animal is not intelligent enough. So it cannot judge the consequences and therefore not consent 'fully'. Many humans also have no idea and get STDs or babies nobody wants. These needed to be prosecuted too, then. Or they will say animals are like mentally challenged people. Then mentally challenged people have just been reduced in status, and also can never have sex. Also, the barrier is difficult to set. Dolphins are quite intelligent. Can they have sex? Is an IQ of 80 enough? 70? 60 vs 59? Or they will say it's like pedophilia. They do this because animals nowadays are completely bambified. Pets are cute. Not fierce. Or there to work. And conveniently it can be linked to something there is no discussion about. However, how is a mature, 7 year old horse that weighs a good fraction of a ton and can punch you through the wall if it wants to equal to a say six year old? One is mature and has a sex drive, the other is not and has not.

They will say animals depend on humans and therefore it is not right, as we exploit that status then. First of all, many people very successfully had sex with wild animals and then people complained, too, cue the wet goddess. And second, if that is so questionable, then most marriages are questionable, as in one way or the other often one part relies on the other more than the other way round. At least for emotional support or something like that. Otherwise there would unlikely be a marriage in the first place.

And then, overarching this, first of all nobody has ever run towards a bull starting on a cow and pulled him off, because "The cow can't talk, it can't consent!!". And second, more than 8 out of 10 people eat meat. I don't think all those dead animals sliced and sold in supermarkets for consumption consented to that. Or to the breeding we did with dogs and horses, to just pic a few species we did this too. Many labradoodles would probably have been preferred to be left alone. Tests for cosmetics are done on animals etc etc etc. Basically, in no other circumstances are people so deeply concerned that the animal can't or didn't consent.

In conclusion: This consent argument is on several levels stupid. The kicker: zoos do obtain consent and have great, mutually enjoyable sex. They don't just jump on a dog that happens to come along or something. Such sick fucks also exist, but that's again the same under humans: just because some are rapists, we don't outlaw all sex 'just to be safe'.