Pope Benedict XVI is like any good professor should be. He just given new insight on something very Darwinian. In a year end message to the Roman Curia, Benedict linked why there are two sexes only and why the environment has to be protected.
The whole Darwinian understanding of sex is premised on the existence of 1) Males and 2) Females. Now it is only males and females period. Either males or females may do sexual selection and reproduce and by consequence any sexual preference aside from heterosexuality (which doesn't result in reproduction) will be naturally selected out.
That is the Darwinian understanding. Any so called "homosexual" behaviour in animals is only an facultative way to achieve heterosexuality.
Straightness rules in evolution and this means a male/female binary only. But some scientists like Joan Roughgarden have provided evidence that there might be multigenders in nature. But Roughgarden has to prove that these genders are functional with respect to reproduction.
The Pope is just reiterating the traditional Catholic teaching but by putting this within the framework of biological diversity , he crossed into very Darwinian territory. His understanding involves the "creating nature of the Spirit"that ensures order in nature and gives rise to biodiversity.
Male and female he created them, Genesis says.
Male and female can only pass on traits and males are inherently superior as a result of female sexual selection, Darwin says.