Yes, Nature can make mistakes, but from the sound of the description, it's more along the lines that the oldest male gets the 'Alpha' treatment, the first-born is the heir to the family, the younger 'heirs' do not have that same level of 'Alpha-intensity'.
That, to me suggest it is more like a familial defence mechansim than an 'error'.
I suppose for it to make sense you have to look back at humanity before we started building cities etc, from a biological point of view, the over-expansion of a single family gene can actually be quite dangerous, inbreeding is a dangerous thing, and in the times of tribal-humanity, travel wasn't widespread enough to prevent it happening.
So, yes, even though I'm the youngest of 3 boys, I think that the more children a female has, in many cases, the later children will not have the intesity of 'maleness', for want of a better word, than the older ones. Indeed, homosexuality may just be Natures way of preventing family gene from spreading too far.
So I do think homosexuality was 'part of the plan', the fact that some people are uncomfortable with it isn't really evidence that it is wrong, in my opinion.