The sight of Stephen Mangan's prosthetic penis in Joe Penhall's Birthday was too much for preview audiences, who thought it was real. Why can't theatre cope with too much reality?

A few weeks ago, the subject of this column was theatre previews: what their function is and whether, with audiences increasingly tweeting and blogging their responses, it's still practical or justifiable to have a run of critic-free performances before press night. And, usefully extending this debate, a production that has just opened in London offers an intriguing example of the sort of changes that can be made during previews. What disappeared was Stephen Mangan's penis.

This abrupt cut was made before the opening of Birthday, Joe Penhall's new play at the Royal Court, which concerns a man who gives birth. As the playwright explained to me during a Radio 4 interview last week, in the original script, there was a scene in which the expectant mother-father had to undergo an emergency procedure to his member, which had become inflamed by the pressure of the baby on the prostate gland.

For the play, Mangan wears a prosthetic false front, which includes hairy breasts and a nine-month baby bump, below which the flesh suit extends into a pretend penis. But, as Penhall told the story, while the audience at one preview reacted with amused and enjoyable shock to the sight of the character's female characteristics, they fell into cold and uncomfortable silence at seeing the evidence of his masculinity. The production was ambushed by this moment.

What the dramatist believes to have happened is that theatregoers enjoyed the overall joke, but assumed that the penis was the actor's own – and so became protective on his behalf, not appreciating that this was another theatrical illusion. My own suspicion is that, as the script specifies that the organ is aroused at this stage, viewers may have believed that they were witnessing something seriously transgressive – the depiction of an erect penis remains taboo in mainstream theatre. (The impotence of the teenage boy in the famous nude scene in Peter Shaffer's Equus was legally necessary as well as psychologically astute.)

Whatever the explanation for the audience's reaction, Mangan's rubber penis was cut from subsequent performances, remaining hidden inside his hospital gown. But it leaves us with an intriguing parable about the relationship between an audience and a play. What they thought they saw of Mangan seems to have pricked the consciences of the viewers, turning what they were enjoying as a fantastical comedy into something too disturbingly real.

I was also struck that, in the same conversation, Penhall made another point about realism and theatre. It would be impossible, he argued, to find a producing theatre and an audience for a play in which a heterosexual couple go into a maternity hospital and are delivered of a healthy child.

I think Penhall is right and the reason for this is that the birth of a child, although ranking among the greatest and most important dramas of many people's lives, is, in theatrical terms, boring: in drama, there needs to conflict, suspense, surprise. For a plausible play to be made from this material, both parents would need to be women or both men or the child would have to be born disabled, dead, with an unexpected racial profile or turn out to be the child of Satan or an alien being. Or – as happens in Birthday – for the mother also to be the father.

The moral seems to be that if theatre becomes too apparently actual or documentary, a contract between production and audience is broken. "Human kind," wrote TS Eliot in Four Quartets, "cannot bear very much reality." As Birthday has shown in various ways, realism, in the theatre, has to be carefully – well, handled.