The kiss, which was not in Ira Levin's original script in 1978, appeared in the film version of the play, and subsequently many directors have opted to include it in their stage productions. The reaction of the audience member to this kiss in Utah in 2014 is not so different from the reaction to the kiss in the film version in 1982. Here's how how I describe it in the book:
The 1982 film version of Deathtrap, directed by Sidney Lumet from a screenplay by Jay Presson Allen, is more explicit and emphatic in its depiction of same-sex desire. The film includes a kiss between Michael Caine and Christopher Reeve, which occurs immediately after Myra’s murder and reveals Sidney and Clifford as conspirators and lovers. The kiss was immediately controversial. Talking to film scholar Vito Russo, Reeve commented: "I heard that a preview audience in Denver booed the kiss, and that was reported in Time magazine, thus ruining the plot for millions of people. We later referred to it as 'the ten million dollar kiss' as an estimate of lost ticket revenue."
The kiss, of course, is meant to surprise and shock at this point in the narrative, but the kiss is also shocking because of the identity of the kissers: two major Hollywood stars, one identified with Superman, an icon of American masculinity. At this time, Hollywood was experiencing an unprecedented flowering of mainstream films with queer subjects--Making Love, Personal Best, and Victor/Victoria were all released in 1982—but audiences generally knew what to expect from these films, thereby containing the threat of the queer. In Deathtrap same-sex desire was sprung on an unsuspecting audience, and many were not pleased. [Murder Most Queer, page 74]
I would argue that the "problem" in Utah in 2014 isn't really the kiss itself, but rather the perception shift caused by the kiss. The homophobic audience member was investing in these characters (as one does in the theater) only to discover at the end of Act 1 that they are queer, and she felt duped, tricked into caring about people that she'd rather didn't exist. So here we are, 36 years after Deathtrap first played on Broadway, and this thriller is still provoking anxieties over the secrecy and revelation inherent in the drama of the closet. Which, in my reading, is exactly what the play intends to do.