If in fact homosexuality is such a great evil, and is a result of human sinfulness, why is it so universally observed in other animals?

You haven’t established that it’s universally observed in animals. You claim that it’s been observed in 450 different species (a tiny fraction of even vertebrate species); you offer no means of checking the validity of those observations.
One behavior that is clearly rampant among animals is indiscriminate sex. Such behavior exhibited by humans clearly stands at odds with their psychological development.
So why did God make animals that are promiscuous? Or, for another example, animals that make messes without cleaning up after themselves? Maybe to give humans an illustration of what the human ideal is not? How uncivilized we become when we totally abandon our sapient dimension and demote ourselves to creatures of pure emotion and instinct?

If looking to the design makes me a Thomist, so be it. We do it with every other organ system; why not the reproductive system?

But as I noted above, it’s absurd to take the position that because an organ is “designed” for one purpose it’s “unnatural” to use it for another purpose as well. The tongue is “designed” for tasting food, but it works quite well sexually, as do the hands and fingers. So does the anus, if that is one’s preference, and there are lots of straight couples who enjoy that (and I presume, some gay couples that don’t). So the equation of “gay” with “anal sex” continues to be problematic, and the concept of condemning any use of an organ for a secondary purpose. Some organs, like the penis, have multiple functions, both excretory and reproductive. And of course, it still remains true that sex is not solely engaged in for reproduction; indeed, it is rarely engaged in for reproduction. Sex also has more than one “purpose” (by which I only mean “useful function”, not “intended function”). It is useful not only for reproduction, but also for building emotional intimacy in a relationship. And of course, one can argue that pleasure seeking is a virtue in and of itself (or at least that it is not necessarily wrong or unnatural to do – no one would argue that taking a hot bath is immoral merely because it brings one pleasure).

Let me see if I understand the logic being proposed in some of these posts: if God exists, then if God created creatures that by their very nature exhibit homosexuality, then God is not inherently averse to homosexuality – thus there is no basis for God to oppose homosexuality in humans.

Not quite. No one has taken that position. But the question has been asked, what is the Christian explanation for this? If in fact homosexuality is such a great evil, and is a result of human sinfulness, why is it so universally observed in other animals? I don’t think this alone tells us all that much about the subject, since there are other traits we observe in animals that we would all consider immoral if done by humans (killing a rival to gain access to females for mating, for example). But so far, no one has even attempted to answer that question.

One of the assumptions here is that animal homosexuality is normal, and not the result of the critter finding itself in heat but, for whatever reason (like the absence of an opposite-sex partner) grabs a substitute (same-sex partner, a human leg to hump, etc.)

This isn’t an assumption, it’s an observation. Many of the observations of homosexual behavior in animals are not instances of merely finding a substitute while in heat due to lack of the opposite sex. Bonobos in particular engage in homosexual acts about half the time, on a daily basis, even with opposite sex mates available easily. And of course, this really doesn’t answer the question because, naturally, God could simply have created animals not to do so if he’d wanted to.

The Bible does not claim that God places any moral expectations on nonsenitent beings. Nor does it claim that the behavior of nonsentient beings is a benchmark for what types of behavior are acceptible in sentient beings.

Nor do I think they should be the benchmark, for reasons stated above. But it’s difficult to make the “unnatural” argument when we observe the same behavior throughout nature. One can still argue that it’s wrong, of course, but not on the basis of it being “unnatural”.

The essential difference between an anus and a cloaca is that you can procreate through a cloaca but not an anus.

But you can both procreate and reproduce through a penis, so the argument cannot be that if an organ can be used for excretion, it can’t also be used for sex. You can’t reproduce with your mouth either, but that also can be used sexually, as well as your hands. These body parts are not like an eye, which only has one function; they have multiple functions. If the argument is that it’s “unnatural” to perform any sexual act that can’t result in reproduction, then I’m afraid most people are going to tune you right out, and rightfully so.

Have you never considered the plight of Sammy Sperm when he thinks he is going to swim upstream to meet an egg, complete his chromosomes and become fully alive, only to find he is in a crappy rectum with not an egg in sight? Not only is there not an egg in sight, not only is he out in left field, but he is not even in the right ball park!

The same is true during masturbation and oral sex, not an egg in sight. Are we going to wax eloquent about poor Sammy Sperm in those situations and declare that masturbators and fellatrixes may not get married? I know this will come as a shock to you, but the vast majority of the human race only very rarely has sex for the purpose of reproducing, whether they are straight or gay. In fact, I have never had sex for that purpose in my entire life. Even those who have children have likely only had sex for the purpose of reproduction a tiny fraction of 1% of the times they’ve had sex. Far more Sammy Sperms die in a kleenex than ever get the opportunity to hunt for an egg.

On the other hand, have you considered the wonders of a vagina?

As often as possible.

And while Sammy is swimming as fast as he can towards his goal, his depositor gets to look at the face of his beloved. What a plan! Don’t you love it?

There’s a doggy style joke in there, but I’m not touching it. Of course it’s true that the genitals work very well for reproduction. But they also have other purposes, as does the act of sex itself.