Czuch:

>  it is a "nature" argument, and not really a "religion" argument

What is "natural" in human beings is difficult to say.  We are very different from other living organisms in this planet.  Most of what we do is not natural in the sense that it is not observable in nature.  For example, it is not natural for a person to sit in front of a television for a few hours.  It is not natural for people to build skyscrapers, or rockets that go to the moon.  Human sexual behaviour is very different to that of other animals.  We are the only species that mates face to face.  I think that mating face to face has sometimes been observed among bonobos (a species of chimpazee).  The old trusty Missionary, the most common sexual position among humans, is uniquely human.

Is homosexuality unnatural? People would be surtprised to know how common homosexual behavious is in nature:

http://en.wikipedia.org/wiki/List_of_animals_displaying_homosexual_behavior

Bonobos (our closest genetic relative) exhibit homosexuality in several forms, although homosexuslaity among females is more common.

To say that prejudice against homosexuality has no relationship to religion is erroneous.  Western culture has been strongly influenced by Judeo-Christian values.  Homosexuality was taboo among the ancient Israelites as exemplified by some of the laws in the Torah.  That same taboo was passed on into the New Testament.

Other cultures outside the Judeo-Christian cultures had different views of homosexuality.  The ancient Greeks engaged in open homosexuality and among Greek aristocrats pederasty was the desirable form of education for young Greeks destined for political and economic power.  Homosexuality was common among the ancient Chinese up to the late Qin dinasty (end of the 19th century).  It was not uncoomon for male concubines to form part of the Chinese emperor's harem.  Homosexuality was acceptable among Sufi moslems in the 18th century, as attested in the poetry of the Persian Sufi poets.

Clearly our western dislike for homosexuality is culturally driven, and western culture is Judeo-Cristian in its origin.  The current definition of marriage is the Biblical definition of marriage, and that definition holds homosexuality as taboo.  If homosexual marriage becomes acceptable, then a lot of people fear that it would diminish the meaning of marriage, and therefore the values expounded by Judeo-Christian culture.