Same gender arrangements between men are immoral. The Talmud says that this is one of the crimes for which the Great Flood was visited upon the world. Thus, G-d expects gentiles to have that much understanding. And if they have created a decadent society that can intellectually argue for the beauty and morality of such relationships, that is mankind’s failing. They were not created that way.
Same gender arrangements between women though, cannot be considered immoral. There is not even a clear Issur for Jews to engage in this practice. The Talmud deems it disgusting and shameful but the courts are not empowered to enforce their separation. Although this clearly demonstrates our duty to shun such behaviour, it can be argued that something that is merely shameful and disgusting for a Jew, without minimizing its gravity, may be acceptable for gentiles.