So many presumptions...
I like the phrase "successful species": would that include mankind, because we have conquered technology? or ants, because if our technology fucks up, there is a good chance they will inherit the earth.
How about lions, who top the food chain in many areas, but are not expecially numerous compared to many other species?
And what is this about the majority of the population needing to be "straight" to prevent dying out. I guess, technically, you may be right, as it helps for most females to be straight...but in most mammalian species, it is the fact that males produce lots of sperm whilst females produce relatively few eggs that is key.
one (biological) strategy, given such a state of play, is monogamy. Another is pack or herd groupings, within which a small number of alpha males ...sometimes just a single one... rise to the top and get awarded the right to shag to their heart's content, whilst the majority of males are condemned to a life of lonely celibacy.
Or shagging other males.
I have no idea whether homosexuality is a "natural" behaviour...personally i find the attempt to split sexual behaviours into natural and unnatural both daft and fraught with bear traps: but there is plenty of evidence of it existing in the wild as a natural response to a given species' breeding strategy.
That said, I am not sure I am aware of any species for which tying their partner down prior to love-making and applying a riding crop to their sensitive bits is recorded as a viable mating strategy.
So moralists may yet take some hope from the biological determinist agenda.