'Normal' is not defined as never, more like usual. And perverted is defined as abnormal, unusual. In animals it is generally a natural instinct to dominate when a male or female acts submissive. That could be interpteted as acting homosexual. I doubt there are many animals that literally don't mate with the opposite sex ever. Do believe that sexuality is a sliding scale for evolutionary reasons, being more or less feminine or masculine can have advantages depending on an environment, which may change rapidly. So varience across a population would be good. But varience based on random gene inheritance could easily lead to a small percentage of outliers that cross a threshold of sexual preference that makes them only attracted to same sex. Probably. Wasn't there a study that showed women that like masculine men are more likely to have gay sons (ironically)? There is also helpful uncle / aunty theory. Someone that can defend the tribe or help with child care. But doesn't consume as much resourse as an extra family. Not sure if the numbers add up on that though.