Because it was written by backwards people from the iron age.
People who otherwise might seem educated and believe in the theory of evolution may imagine that backward people would be modest. However, I feel that the complete opposite is demonstrated. The closer one is to the slime pit (according to a theory of upward and uplifting of society and the evolution of man) one should see that childish behavior among adults should cease all together.
In other words, if evolution is in fact true------man should become more and more MORAL He should see any deviant behavior as a throwback to the apes and animals of the past. Sex should become more and more only a means of procreation when necessary.
HOWEVER, what is observed is that man is becoming more and more of a beast and enjoying rolling in the slime without desiring procreation. And this would point potently to the reality that man is devolving, and becoming more and more like the generation that Noah lived among before the Flood.
So, homosexuality and its acceptance by society, is in actuality demonstrating that man is digressing and not improving. He needs salvation, and his denial of it only turns man into a beast of the field that presently still is capable of creating toys to humor himself but not improve his destiny.