This debate ends NOW.

For long, conservatives have labelled homosexuality as being 'unnatural'. The arguments are:
- there is no evidence of any 'gay' genes.
- there are many cases of gays 'turning' from gayness.
- therefore, it is, like most sexual behaviours, a preference and only a choice.

On the surface, those arguments may seem compelling (especially the first one). However, conservatives have ignored (and liberals have never mentioned) one truth: that homosexuality has been extensively documented in hundreds of animals.

From wikipedia:

Homosexual behavior in animals refers to the documented evidence of homosexual or transgender behavior in non-human animals. Such behaviors include sex, courtship, affection, pair bonding, and parenting. Homosexual and bisexual behavior are widespread in the animal kingdom: a 1999 review by researcher Bruce Bagemihl shows that homosexual behavior, has been observed in close to 1500 species, ranging from primates to gut worms, and is well documented for 500 of them.^[2][3] Animal sexual behavior takes many different forms, even within the same species. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied.^[4]

The naturalness of homosexuality in non-human animals is considered controversial by conservative religious groups who oppose LGBT social movements because these findings seem to point to the naturalness of homosexuality in humans.^[1] Whether this has logical or ethical implications is also a source of debate, with some arguing that it is illogical to use animal behavior to justify what is or is not moral (see appeal to nature).

(my emphasis)

Of course, in this case, progressives aren't arguing that homosexuality is necessarily moral-merely that it is natural.

The only counter-argument against this is that in the animal kingdom, homosexual behaviour can be learned from the parents (if they are also gay). Certainly, there is evidence for this; Roy and Silo, two gay penguins, raised a chick that was bisexual herself. Also, bonobos (a species of chimpanzee) are a bisexual species, as all bonobos have been documented engaging in sex with males and females. These accounts lend to the theory that homosexuality in animals in more learned from observing the parents (which is how almost all animals learn) rather then it being innate.

However, this fails to take into account the long-term affects of a nurture-only behaviour in an environment governed by evolution. If animal homosexuality was merely a sexual behaviour that animals mimicked from their parents, then over time these animals would reproduce less then their heterosexual counterparts. For example, bonobos practicing heterosexual sex more often would reproduce more and their children would be more likely to mimic the heterosexuality of their parents. Thus, the behaviour (still being passed down from parent to child) would pass down less times until it would become an extinct.

Can this argument be extended to nature? No. It is true that by evolutionary standards, 'useless' biological components that make up homosexuality wouldn't be able to compete on a mating level with hetrosexualism. Therefore, the 'gay gene' would eventually in members of a species would decrease over time, until it would disappear altogether. However...

...this ignores the reality of mutation-the basis of genetic diversity. In the most blunt layman's terms, a mutation is when a gene of our genetic code changes, causing changes in the cell. If the mutation occurs early enough in the womb, the mutation can affect the entire organism. Mutations can range from being relitively minor, such as syndactyly ('webbed' digits) to being quite major, such as growing extra limbs. Naturally, not all mutations are as probable as one another. A common (and infamous) mutation is cancer. In cancer the gene that regulated when a cell will reproduce no longer functions, and the cell's reproduction becomes uncontrollable. At the other end of frequency, the aforementioned 'extra limb' mutation is extremely rare and is considered newsworthy.

If such genetic diversity can exist to create whole new limbs, then it is no a stretch for this same genetic diversity-based on mutations of the genetic code-to cause a range of sexual orientations.

Hypothetically, it is possible for some animals (especially the more intelligent ones, most famously bonobos) to indulge in homosexual activities for pleasure or experimentation, without any influence from parenting. However, this cannot account for the extraordinary number of gay animals-at the absolute least 500, and almost certainly over 1,000. In particular, insects have been identified as being gay (or at least, engaging in gay behaviour). Due to the extremely primitive design of insects' brains, is is extraordinarily unlikely that these animals would be seeking out more pleasurable behaviour.

If then, homosexuality is natural, then there is some discussion as to why some people can become 'ex-gays.' Unfortunately, a lack of neuropsychological studies on the impact of sexuality on brains means that it is difficult to assess the extent to which some ex-gays have genuinely changed their sexuality, and how much of it is internal denial. However, the argument for internal deniel is strong, according to this.

Exodus describes change as "attaining abstinence from homosexual behaviors, lessening of homosexual temptations, strengthening their sense of masculine or feminine identity, correcting distorted styles of relating with members of the same and opposite gender."^[21] Some ex-gays advocate entering (or remaining) in a heterosexual marriage as part of the process. Some in opposite-sex marriages acknowledge that their sexual attractions remain primarily homosexual, but seek to make their marriages work anyway.^[22] The president of Exodus said he agrees that people can't necessarily change their sexual orientation, but he said they can "live in accord with their beliefs and faith" by renouncing homosexuality and not engaging in same-sex relationships.

(my emphasis)

This is, by the president's own admittance, a very low bar being set for the term 'ex-gay.' It also means that 'ex-gay' is a misnomer. Change and ex-gay imply that you were formally gay, now straight-not still gay and simply repressing your sexuality. The APA has also stated that despite the "considerable fluidity in...women's attractions," "human beings cannot choose to be either gay or straight." This means that any percentage of the ex-gay movement-from 1% to 97%-could still be gay, and effectively living a lie.

Conclusively (in my opinion, anyway), this post has demonstrated that homosexuality occurs naturally within the animal kingdom, and across a tremendous range of animals-from bonobos, who possess a spectacular genetic similarity to humans, to birds, to insects. Exodus' open admittance that some gays cannot change their sexuality leads to the 'nature' conclusion.

Also, see here, here and here.