There is a huge chasm between "documented" cases and your word "natural".

Odd how you put out "500 species" but you didn't give the reader "the base rate"...that is the denominator, on a basis, in other words, of what number of species? 

How many species are there? 8.7 million.

To refrain from causing embarrassment, I'll let the reader do the math with your 500 number pasted from Wikipedia. 

And this number needs to be further weighed and compared, given the number of "hetero" copulations that are occurring even within those 500 species. 

That would be a marvelous number to know..."out of all the copulations from all the species what percent are homosexual!!!!!"

Such anomalies are not natural.