First, what exactly do you mean by "homosexuality"? Is it only physical acts? Is it only emotional feelings? A mixture of both?
As has been said, different cultures today and throughout history have had different social standards when it comes to sex, gender, and sexual orientation. What we consider to be "homosexual practices" are not necessarily the same as another culture.
My personal opinion based on my own reading:
The accepted physical acts and relationships between people are defined by the society the live in. But the internal feelings and preferences of the person are probably determined by an interaction between genes and the environment they are raised in. By this I mean that two people with identical genes would not necessarily turn out to be the same sexual orientation or gender when raised in different environments. (Here gender means a societal role that people of either sex can fit into, with sex being determined biologically)
Also, you really,
really need to make sure you take anything in ancient history within the context of that time period. For example, sexual orientation wasn't even the same concept in ancient greece as it is today:
http://en.wikipedia.org/wiki/Homosexuality_in_ancient_Greece
The ancient Greeks did not conceive of sexual orientation as a social identifier as modern Western societies have done. Greek society did not distinguish sexual desire or behavior by the gender of the participants, but rather by the role that each participant played in the sex act, that of active penetrator or passive penetrated.[5] This active/passive polarization corresponded with dominant and submissive social roles: the active (penetrative) role was associated with masculinity, higher social status, and adulthood, while the passive role was associated with femininity, lower social status, and youth.[5]
Pederasty in ancient greece and rome is NOT the same thing as homosexuality is today. In fact, one of the concerns of ancient greeks was that the
eromenos, the youth, would become a
kinaidos, the passive, penetrated partner in adulthood, which is what women were. So even though sexual relationships between members of the same sex were allowed (homosexuality in today's world, or at least here in the USA), and even encouraged in certain circumstances, there was still worry that the boys would turn into men who preferred to be "submissive" and "passive", instead of dominant like adult men should be. Homosexual relationships between adult males of similar social status were generally not approved of.
Homosexuality has less to do with the physical acts, and more to do with gender roles in society. It's more about masculinity and femininity (gender), and less about male and female (sex). Hence the stereotype of the gay man who acts with a lot of femininity, or the butch lesbian. I'd propose that most people who are uncomfortable with gay people have more of a problem with this reversed gender role than with the actual physical acts.
(The different uses of gender and sex are merely one use of them. The way I use them is not meant to be argumentative, but to clearly differentiate between the physical and emotional/behavioral differences between people. People are born male or female, but masculine and feminine features and behaviors vary widely between different people of the same sex)