n1r4vHopefully you're at least aware that there are masses and masses of research papers which attribute homosexuality to some genetic or predisposing factor, which it seems you are. However your rebuttal of this is that "t's not necessary to have such high qualifications in order to understand the simple fact that correlation does not imply causation.". Firstly, yes, there may be a correlation instead of a causation, but similarly it may be causation rather than correlation. The fact that there are so many research papers postulating this means that it's highly unlikely that each and every single one of them which states causation, is wrong. Secondly, I don't think you understand how scientific research papers work, and I doubt you've ever seen one or at least found the structure of one, in your life. The foremost element of writing a research paper is showing that your results bear significance and there is apparently causation rather than correlation. That's the main thing that they try and do! You seem to think that you've made a relevation in that these scientific papers might just be inaccurate because it's all just correlation rather than causation. But that's the first thing in their minds after scientists obtain a successful result. They try and eliminate all bias, all confounding factors [which cause association instead of causation] and do statistical tests to ensure that the probability that the results were due to chance was so low, that it could only signify some type of causation, be in direct or indirect. So no, you're wrong in saying that "it's correlation instead of causation" because firstly there must be a few which are in fact causation, which instantly nullifies your argument and secondly, their key aim is to show that the results were not due to correlation, but causation.
Secondly, this assertion "Whether it's natural or not is a completely unrelated question." is quite annoying and I think almost all TSR members who frequent these types of discussions, especially those on evolution and the progression of natural selection, would strongly agree with me. What on earth do you mean by natural? How can you think that nature is some kind of sentient entity with an imperative, objective or even a mind, which directly targets its function towards a particular aim? The idea seems ludicrous. How can you think that it actively strives towards a particular endpoint? That argument seems to be verging on intelligent design which smells like a religious argument, and while that may be a valid argument elsewhere, you seemed to be discussing the intrinsic features of nature, and hence any religious argument is irrelevant here.
In any case, I'll try and use the principle of charity with your arguments. The only way in which something can be seen as "natural" is if most of the human population possesses some kind of trait, and hence those who are "unnatural" and ones who deviate from the normal range of the trait. There are two problems with this. Firstly it is making an "ideal from an average"; i.e you are taking the average or norm and turning this into an ideal, or an optimal trait. Not only is this intuitively unfair, but it is contestable on every level and since you look like you're a philosopher, many of them have argued against this trend, the most popular of which is Hume. Secondly, there is a major problem which "unnatural" traits which confer some positive benefit. For instance think of an animal or in pre-agricultural times, even humans, who were extremely unnatural because they have unbelievably strength or speed. They would be deemed unnatural, however since they would be selected for due to their enhanced abilities, their "unnatural" traits, over time, would tend towards being natural. So there is another flaw.
So I think the only possible way that your statement could hold [I'm using the principle of charity to a major degree now], is if you're saying something is "unnatural" if it confers some kind of negative feature on the human / animal. I.e you're saying that homosexuality is unnatural because it has a major disadvantage of at length, not being able to propagate the genes. This could easily be nullified by a previous point I made about nature not having a particular objective, but I'll ignore that for now. So the only way your argument can hold is if something is unnatural because it confers a negative feature on the animal. Let's take extremely long legs. That would be a major disadvantage in some environments but in others it could be extremely positives. I can't be bothered to give more examples, but it is easy to contrive situations where certain features can be positive in one environment but detrimental in another. Maybe homosexuality was important in some environments in the past. For instance, especially in many animal species that homosexual animals act as a guardian of a certain community/ population of animals. Also I think there was a theory which stated that the gene which was a possible contributory factor in homosexuality was the same as the one which somehow improved pregnancy, which explains why it may have been conserved.
So many possible explanations, but the key one is that assuming that this is a non-religious argument, you cannot possible state that it is not "natural", because it is assuming that nature is a sentient being with some form of higher organisation or a mind, which tries to enact its objectives, deviations from which, are "unnatural".