Similarity
The argument many religious people use against homosexuality is that it is not physically reproductive. This is obviously a good argument. We don’t exist outside of our bodies in any obvious way, and as far as we can tell, everything we do is downstream from our genetics and evolution. It makes sense that we would evolve a physical revulsion for behaviors that decrease our evolutionary fitness and that the same force would apply to our social institutions.
Ultimately this is a reflection of a higher ideal of productivity. When two things come together that are exactly identical, nothing new can be created. You need yin and yang. When you look at it that way though, the religious people probably aren’t going far enough. What is more gay than a man meeting another man you get along with and asking implicitly if there is a more submissive version of him available? When you do that, you enter into an intellectually or spiritually homosexual relationship.
Those relationships are the same as physically homosexual ones in a more subtle dimension, and the outcome is the same in that dimension too. You don’t reproduce the principals that motivated the founding of your organization. That is why organizations decay more quickly when they don’t build in mechanisms for fostering the right kind of diversity.
But also, at the end of the day, humans are so similar to each other that we aren’t really capable of anything other than a quasi-homosexual relationship with each other regardless of our choice of mate. That is why we are starting to look for and create new kinds of intelligences.