I assume you are talking about the US constitution? I know of nothing in that document that protects sexual orientation. The 14th amendment notwithstanding in application really needs to be stretched to cover gays.

To be honest, the laws are going to be tested when animal protection "has gone too far" and the livelihood of meat producers, breeders, and others who make money off animals is threatened and not before. When we start giving animals "sentient" status like they have some in Europe. In the end they will make up some pre-text about consent rather than give the animal an ability to choose.

What is an acceptable use for an animal? Unfortunately that will be the way the laws will have to be worded to be fairly broad and inclusive. I think it could be argued that a law specifically banning sexual contact while allowing other things could be challenged. It would take a lot of guts. I think we have seen several examples of people "not being able to be charged" under current animal protection laws because it just couldn't be proved that the animal was ever harmed in the act.

Unfortunately, because of a lot of reasons, sexual orientation never made it into the Canadian constitution either. I doubt it will ever be made legal here, but that perhaps some understanding of when it is appropriate to enforce might happen?

Interesting side note, it is illegal to have anal sex in Canada with another consenting adult if someone can see you, but not if you are alone with that other person. That includes married couples.

Edit: added details

From my understanding bestiality laws in the United States, are decided by states, not the federal government. Therefore it would be EXTREMELY unlikely to make it to federal courts, let alone the Supreme Court. Searching for the word "horse" on the court's website resulted in almost nothing involving a horse in any of the past cases. I hope I'm wrong and the justices are pretty old (EDIT the law clerks their are fairly young but the former clerk I met personally was old but it was decades since he was a Supreme Court of The United States Clerk), so you never know. In the United States the Supreme Court decides what cases they hear. They only hear a fraction of the cases presented (don't know how it is done in other countries, I only know what I know because I had to take certain classes in college/University to graduate).

Sorry for the double post, but to make it clear, the current Status Quo is that Bestiality should be regulated by state governments. Sort of how Fireworks are allowed in some states but not others. One could say fireworks celebrate the birth of our nation, but the state courts decide not the Supreme Court.

The short answer to your question is that I think these laws are unconstitutional, but it would difficult to demonstrate in court and depends on the sort of scrutiny Lawrence v. Texas upholds, which is debatable. In Lawrence v. Texas, the Texas sodomy laws were overturned largely because they violated a right to privacy inferred from certain constitutional amendments, which the majority ruled extended to a right to private consensual sexual relations between adults. Because constitutional rights and state and federal laws come into conflict all the time, the courts in the U.S. have established three different standards for whether such laws are constitutional. Note that in all three cases, there must be a government interest served by the law in question:

• Strict Scrutiny: Used when the right in question is fundamental according to the Supreme Court or when the law involves a suspect class (for our purposes, a disenfranchised minority, though there may be more to it). In this case, there must be a compelling government interest served by the law, the law must be narrowly tailored to meet that interest, and must be the least restrictive means of achieving that interest.

• Intermediate Scrutiny: Used in cases with a quasi-suspect class (not suspect perhaps by definition, but granted similar status in law). This requires the law to further an important government interest in a way substantially related to that interest.

• Rational Basis Review: Used in all other cases. Only requires that a law be rationally related to a legitimate government interest.

The problem is, Lawrence v. Texas doesn't really define what level of scrutiny should be applied for the right to privacy, and the law as it stands now does not make clear whether sexual orientation is a suspect or quasi-suspect classification. This means that the way the law now stands, rational basis review would likely be applied, and all the government has to do is say "This law exists to protect animals.", and it will probably be upheld. The same is true for intermediate scrutiny, I think, since anti-zoo laws are substantially related to that interest. Our only hope appears to be strict scrutiny, but the courts are reluctant to rule in a way that makes the right to privacy fundamental under force of law or makes sexual orientation a suspect classification.

In any case, for a constitutional case to be made against anti-zoo laws, there'll need to be either some change in the courts' views on the level of scrutiny to be applied in similar cases, or a vast amount of new information on both zoophilia and the effects of these laws. Either way, I'm not holding my breath.