Hello folks New here.
I have to say Wade, I appreciate your wanting to approach this from a secular angle. Approaching it from a religious standpoint is absolutely ineffective for anyone who doesn't already share your beliefs, so if your aim is to make a difference for others, you're on a good path.
As far as homosexual activity being weeded out of a population based on natural selection, there is some evidence to suggest that due to our particular breeding strategies, there could be an evolutionary advantage to homosexuality. The care that goes into raising young is directly proportional to how much resource is required to raise it, and how many offspring the animal has. Mosquitos have thousands of babies in their brief lives, and do nothing to protect them after laying eggs. Sea Turtles are the same. They lay eggs and leave them, and if the eggs are threatened, the mother turtle won't protect them (the idea being that since she can lay hundreds more, she is more valuable in the quest to pass on her genes than a few eggs). We as humans spend enormous resources on each child we have, and we have very few in comparison to many other animals. Since we have a gestation of 9 months, since our babies take incredible resources for us not only till birth, but for years afterwards, it is vital that they are perfectly suited for procreation when they reach maturity. One theory about the advantage of homosexuality is that it provides greater care for the other children, that having a gay uncle means more efficient and effective child care for others. This may be why homosexuality is seen in a number of different species, but always species which have relatively few young, where child-rearing is extremely important.
As a reminder to some who have commented on this thread, the important part about natural selection is that the GENE is passed on, not necessarily the individual. So for instance, some marmots will exhibit altruistic and self sacrificing behavior in order to save the whole family. This would typically be disadvantageous if the individual surviving was the important part of natural selection. Since its the gene that needs to be passed on, self sacrifice is seen in animals who's close family is threatened, but not if its other random marmots. If you'd like a more thorough explanation about the theory, Richard Dawkins wrote a book about it called The Selfish Gene in 1970 something. Its a good read!
This is just one theory of why homosexuality might be a perfectly natural phenomenon.