Consider the ancient world - pederasty and sex with males of a lower social class (slaves) was an accepted part of society, yet anal sex with men of equal age and standing was looked down upon. Why? Because being penetrated was seen as being debased - made passive and womanlike. This did not apply to youth (pederasty) or the underclass because they were not seen as people of equal standing in the first place - thus, to penetrate them anally was OKAY and actually reaffirming the masculinity of the active participant.
When homosexual behavior started to be condemned by the emperors of Rome, who historically took on male lovers, upon the adoption of Christianity, it was decreed that passive males were to be burnt at the stake. Note the word: passive - there was nothing to be said of the active participant until nearly two-hundred years later!
Fast forward to the modern age. In retrospect, this idea seems a little silly, right? "As long as I'm not the one getting screwed, I'm not gay!" Well, it carries on into this society today. Homosexuality and the feminine man are intrinsically linked. See: the flamboyant stereotype, the misconception that homosexual men think of themselves as women...
Or in McCaffrey's case, her association of anal penetration with being an effete man lover. She's just another instance of a long held belief that has more basis in cultural biases than in biology!
sleep depraved speculation
some more information : [http://en.wikipedia.org/wiki/History_of_homosexuality]