The rest from Mercator-net on this topic is well-worth reading:

Why does the same-sex marriage debate seem so futile?

…Are they really “born that way”? Most supporters of same-sex marriage assume that homosexuality is as genetically determined as skin color. Gays and lesbians were born that way and cannot change. Discrimination against them is as unjust as racial discrimination.

However, there is no settled science on whether homosexuality is hard-wired in one’s genes, determined by childhood experiences or a matter of choice. Even the American Psychological Association – which supports same-sex marriage and same-sex parenting – admits that the cause or causes of homosexuality are extremely murky:

“There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors.”

In any case, genetics does not determine moral value. There is a genetic component to cancer, but cancer is not good. There may be a gene for alcoholism, but drunkenness does not excuse unruly behaviour…