More generally, though, there is the question of credibility. Novelists can tell me any sort of nonsense about the workings of the Moscow underground system or the administrative records of a police investigation and I will believe them as long as it sound as if they know hwat they are talking about. I've been told that Robert Pirsig's Zen and the Art of Motorcycle Maintenance is a completely unreliable guide to motorcycle maintenance. But if someone is writing about an area of which I have experience, I can check whether they are not just plausible but authentic. So a different level of scrutiny is applied.
There is a problem here for the writer writing for the naive reader: it will be assumed that any experience described realistically must be real. Kingsley Amis admits to abandoning a novel with a first-person gay narrator because he didn't want his readers to speculate about the extent to which it was true. This seems a bit bizarre: one assumes that Thomas Harris is not suspected of being, or even wanting to be, a cannibal.
Then there is the question of language. Preferred terminology for body parts depends on the writer's (and reader's) age, gender, nationality, class, sexual orientation etc; use of what seems natural for the writer may have an adverse impact on some of his or her readers. For example, Martin Amis' reputation a a misogynist writer incapable of creating a convincing female character may be partly derived for his preference for terminology which is typically male (it is also partly derived from his inability to create convincing female characters: it is notable that the two most fully realised, Nicola Six in London Fields and Mike Hoolihan in Night Train, are cop-outs because Amis explicitly says that they are 'male' psychologically).
And there is the wider question of the extent to which one wishes to be seen to be writing pornography. Somewhat bizarrely, 40 years after the Chatterley trial, using Lawrence's terms in literature would be seen as rude if no longer shocking. To retreat into medical terminology runs the risk of making the act of love sound as exciting as a computer program. Since sex is 90% imagination and 10% friction [source unknown], most of the time writing is about the quality of the activity as it is experienced, and is as much about emotion and attitude as it is about mechanics. This is I think why so much writing about sex is flagged as being bad, in the sense of pretentious or over-ambitious. Even clever writers like Nick Hornby these days steer clear of anything hinting at high style: simple words in simple order are the norm. Purple prose is something of an endangered species in modern literature (with good reason, of course).
There is a danger, though, that being overcritical of the attempts to address the subject will lead to the easily-swayed from avoiding it altogether, leaving us with a mechanical prudishness at the core of fiction. Sex is important as a way of revealing character and a way of communicating mood, and on the whole writers should be encouraged to attempt its description, even if some are bound to fail.