The last 20 years have seen a marked rate of change in the public acceptance of homosexuality as a normal part of society.  This is stark contrast to the prior 1000 or so since western european cultures beginning with the Gregorian Reforms of the Catholic Church.  'The first ecumenical council to condemn homosexual sex was Lateran III in 1179.  Even earlier Roman Empire law promulgated by Justinian in 529 made it a crime punishable by death unless repentant, though it was still widely & openly practised thoughout roman society. 

The history of ancient cultures throughout the globe shows they practised homosexual relations and had no concepts of the modern "homosexual" / "heterosexual" division. Notwithstanding, however, ancient Greek philospher's were varied in their view of homosexuality... Plato for example in earlier writings condones it, while in later ones he opposes it.

The point is that homosexuality and same - sex attraction has been a normal part of most cultures over a variety of times as best historical records have found.  In some cultures and at some times it was promoted and condoned, while at others condemned and persecuted.

This is not to say that normal means a majority of the population practised homosexual relations, but that a substantial proportion of society engaged in it... temporarily or more/less permanantly without persecution or social malace. Thus, it is natural part of the spectrum of human traits, independant of time and culture.

On thinking about this it stands to reason that it's a natural and normal part of the human spectrum.  We accept as natural that a proportion of humanity is more passive; that some are naturally more aggressive "by nature".  We accord with equal ease that naturally some proportion of humans are more introverted and some extroverted.   We also see and accept as natural the physical traits associated with different races --- hair/eye colors (some dominant & some recessive in hereditary selection); facial features, body type, etc.  We accept as natural a diverse spectrum of cognition (aka intelligence); of some proportion of people that are more "artistically inclined", as opposed to "analytical" (i.e. also termed right and left brain dominance).

Since these diversities exist naturally .. i.e. tendencies & prevelances created by one's dna, then it follows that these diversities are a pure function of random or geneological probabilities inherent in the formation or growth of the zygote... embued & fixed at time of fertilization or during the early processes of cell divisions which ultimately result in a zygote, then fetus, and finally a birthed human. 

Why then, should sexual preference diversity be any different?  We've all known or interacted with people we immediately recognize as "behaving" differently from the time we were children   We may not have recognized the difference then as having sexual preference differences, but it became clearer as we got older that some males were more "feminine" in behavioral traits, and some females were more "masuline" in those traits.... despite the fact that while we were young adults it was socially and religiously unacceptable to openly show these traits in terms of sexual preferences.    In other words while heavily socially, religiously, and even legally suppressed, these traits were never-the-less still apparent to us even if not openly ackowledged. 

In short, we see both in our own experiences & observations, as well as verified by historical records that homosexuality has been a normal part of human traits and one of the many diversities among humankind.   Why then should it have been and/or be considered an "unnatural" human behavior?  More importantly perhaps than it being "unnatural" is that it is has been and / or is considered "not normal".

"Not normal" has many different connotations --- being a minority is "not normal" by definition if "normal" is a given minimum plurality of the whole.  Is 10% of the whole part of the "normal"? or is it "abnormal"?  What about 20%? or 5%? 

The normal distributioin of human traits follows a probability distribution similar to a gaussian probability distribution which is also termed the normal probability distribution, or normal curve for short.   As such,all humanity is resides somewhere inside the normal distribution.. .therefore "normal".  What society does however, is ascribe "abnormal" to those smaller proportions of the distribution near or within the tails of the distribution.  For example the normal measue of IQ's, regarless of  method of intelligence measure are follow a normal distribution... the distribution is arbitrarily assigned a value of 100 at the mean and the variation about the mean (standard deviation, s, or sigma), is scaled so that 15% of the IQ's fall on either side of the mean.. i.e. 1 sigma = 15. 

So the normal distribution of IQ's for 99.86% of the population fall between a low of 55 and a high of 145.  But we accept as "normal" only the center 1/3rd of the poplation of intelligence --- from approximately 84 to 116 or so.  Those below 84 are considered "dull", or "mentally deficient",even though they constitute ~ 1/3rd of the population.  Those above 116 or so are considered "above average","superior", or "genius" to varying degrees and thus "not normal" but yet they also constitute ~ 1/3rd of the population of human intelligence.  

When it comes to sexuality or sexual preference society has sought to suppress experssion of preference so that it appears on the surface to be a very rare, "abnormal" condition. But as has become more and more apparent, if society can no longer create suppression, the the proportion of the population expressing a same-sex preference is much, much greater than it appeared to be under effective suppression. 

The proportion of US gay population estimates are from 1% - nealry 6%, but yet in SF and Seattle the gay population represents 16% & 13% respectively.  (source).  That's a big difference... so it's not clear that social stigma or persecution isn't still highly suppressing open acknowledgement of alternate sexual preferences. 

In any event, some proportion less than 15% would still be considered "abnormal" in the sense of being a small minority of the population.  But the "abnormal" classification status is a social moral classification ... with negative connotations.. For example it wasn't too long ago that only 10% of the population graduated with a degree from a 4 year college or university, yet college degrees were not considered "abnormal", rather they were considered "exclusive" ... i.e. given a positive connotation.

Similarly we glorify "genius" but condemn the "imbicile", while both are relatively equally rare in the population. [Imbicile in popular definitions has IQ below Moron (59-69 IQ) , but above an Idiot's (<29 IQ) ] (source) .  

Societies generally condemn the proportion of the population that is "diffferent" ... so racism exists --- blacks in the US, whites in Japan both suffer of effects of racism by the respective majorities, for example.  So it is not unusual for society to condemn the minority of humans with sexual preferences different from that of the majority.

But historically it has not always been the case that those with a homosexual preference were considered a minority or were even in the minority... so that the current effect in western societies is one that has been generated by a religious moralisms.... not unlike religious moral attitudes against sex before marriage, or sex for other than procreative purpose.

In short homosexuality is a normal non-deviant behavior of some proportion of humanity, dictated by a genetic or dna related phenomena of human traits.  It's condemnation stems only from a belief system that condemns "difference" when the difference provides a perceived threat to the beliefs of the majority.  Most or all of western religious morals have been arbitrarily assigned by religious leaders in the early years of christianity's rise.  Not at all surprisingly these moral impositions include condemnatioin of behaviors of the minorities.... equating the behaviors with "sin".  Interestingly, the foundations of Christianity lie in the old testament and in which the ten commandments didn't identify homosexual preference or acts as a condemned act or behavior.... and it isn't even clear in the old testament that homosexual acts were socially or morally condemned.  The foundations for opposition to homosexual behavior and acts therefore lie in times and cultures where western religion provided a basis for condemnation. 

Maybe sexual preference is really just something like epicurean preferences --- I like fatty red meats, for example, strong red wines, but have a strong distaste for peas, cranberry sauce, &  white turkey & chicken meat.   Or maybe sexual preference is just a sub-set sexual preferences in general --- I prefer female brunettes to blonds for example... that's also a sexual preference.  

 Terman's Stanford-Binet Fourth Revision classification