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I guess I'm of the opinion that being gay and straight is exactly the same, minus the "who" part of being attracted to people. Homosexuality can be found in nature, and even helps out populations. For example, the laysan albatross has a much higher hatchrate for females than to males. So often, the female birds will raise chicks together. Some gay penguins from various species have adopted abandoned eggs and raised them together. Lots of gay couples adopt and raise children, so I don't think being gay is at all counter-productive to the continuation of our species. Reproduction might only be possible between straight couples, but giving birth isn't the only thing you need to do to keep a species from dying off.
It's impossible to know "why" animals or humans might be gay, and you're right in that there's no conclusive evidence, although it's generally understood in psychology that all behavior is rooted in a combination of genetic predisposition (regardless of whether we know which genes contribute to behavior) and nurture/environment. It's not some weird phenomenon either. Much of how our brains work/why certain behaviors happen still isn't known, and scientists are learning more about our bodies all the time.
So we might have to just agree to disagree. I'm strapped for time atm so I can't type much more. As for connotations, they are part of how words are defined and important to language.