Any form of sex cannot be considered unnatural for the very reason that sex always exists in nature. In fact, nothing can be considered unnatural since everything is inclusive to an absolute extent under nature.
People develop a special interest about what goes on in other people's beds because sexuality is defined by culture. Copulation is a pretty central part of the human experience — if not the most important. The rejection of homosexuality from some groups is primarily based on the lack of offspring production. Even so, this perspective only provides a small part of the whole picture.
There is a failure in understanding that the production of offspring involves quite more than the processing of inserting a penis inside a vagina. Human societies for hundreds of thousands of years used to exist in communal forms. Small groups of males and females gang-banged together and created large families. Homosexuals assisted with the upbringing of the children and household chores. They were an integral part of the family as much as any other member.
Being gay only got a bad reputation after the arrival of monotheistic religions that insisted on nuclear families between one man and one woman. Nonetheless, the invention of marriage was perhaps the most successful business idea of all time. In the past, only a few elite got to breed with as many females as possible. This is how things rolled from hundreds of thousands of years in our species and this is part of the reason the human populations were kept in relatively low numbers.
Similar patterns can be observed with other mammals. The "alpha" copulates with as many females as possible while the rest are assisting with the hunting. Some would still snick in a fuck or two but the general sentiment was pretty much the same. A few alphas enjoyed most females while the rest of males just stuck with the dick in their hands. What was "natural" then, it is not "natural" today.
Marriage became a rather successful enterprise for state religions. It was an attempt to increase numbers, influence and power through an undeniable "holy" dogma and it worked since everyone was getting laid. Following this cultural change there was an additional reason for meaning in life. People with families had purpose for sacrificing themselves in battle, thus ensuring resources for their offspring. There was no need to force people into self-sacrifice for the "glory of the King". The social memes of patriotism, religion and family secured the whole conquest enterprise together. That was "natural" as well.
Whether entertains the idea or not, sexual behavior has a lot to do with time and place. If one would find themselves born in Yemen, it will evidently appear completely natural to marry a 13 year child. Nature doesn't disagree with the premise since a human being is pretty much ripe for copulation at puberty.
This is also the reason the age of consent varies massively across different countries. Based on the same social imperative some groups of people homosexuality as natural and some do not. In a similar respect, some people find it "unnatural" and "unethical" for an older male to be with a female in her early 20's. In the past some people found it unnatural for black people to drink from white folk fountains. Thing is, mother nature doesn't give a shit about what people call natural or unnatural. Mother nature expresses itself through it's living subjects, not vice versa.
Mother Nature is pretty wise about what allows to be propagated through the gene line. Homosexuality is no more unnatural than an elite athlete. Rareness in behavior, taste or gene expression cannot be an indicator or naturalness. Ecosystems, which are the hear and soul and of Nature thrive or variations and suffer under homogeneity. That alone should be able to answer to everyone whether being gay is natural or not.
Contextual References:
[1] Jezebel.com: Men's Favorite Ages Are 20, 21, 22, and 23
[2] Wikipedia: Age of Consent
[3] BBC.co.uk: The Origin of the Human Family
