Easy. Pick an animal that has stayed relatively the same for millions of years and observe it. Its behavior will not be any different especially in something like a DragonFly or a Whiptail Lizard or Shark.
Nothing easy about. We've only been studying animals for a relatively short amount of time, and their behaviors for an even shorter amount of time. In fact behavioral changes in species may very well be the reason that they've changed or haven't changed overtime, as opposed to environmentally adaptive changes. It's one of the fundamental flaws of predictive behavior.
When getting my undergrad degree, a good example was butterflies changing their mating seasons to overcome predation, leaving them physically unchanged.
Fact remains, behavior of animals millions of years ago is nothing more that speculation. Observed behaviors in the present day may very well have not existed millions of years ago.
It's speculation, and you know it.Of course you wouldn`t even have to do that as you could observe an animal, its mating habits, ecology and then compare with its closest ancestor and do the same and so on. You could even anaylze at the Molecular level and compare DNA and divergence of living or even extinct animals. We are all cousins of every living thing you see on earth and share the same DNA. The more similar the DNA, the closer we are related and the less and more recent we have diverged.
You do realize that Humans lived at the same time as Many other species of humanoid (Homo) Such as Neanderthalensis which we could not mate with and which did outnumber us initially until they died out. They were nearly as intelligent as we are now and we both competed for the same resources. In fact, their large brains and bodies needed around 4000-5000 calories a day and once we encroached, they could not sustain themselves whatsoever as we gobbled up their resources and hence we watched them die out so to speak.
None of this has anything to do with what we're talking about. You're just attempting to muddle the issue.
I'm a staunch believer in evolution, so spare the bullshit. The issue is behavioral speculation of species millions of years ago, and at this point in our understanding it's just that - speculation.
You can offer absolutely no scientific proof that homosexuality has existed for millions of years.
And aren't you the one always droning on about how there's only fact vs no fact?