http://en.wikipedia.org/wiki/John_Wayne_Gacy#Murders
"On December 30, 1977, Gacy abducted a 19-year-old student named Robert Donnelly from a Chicago bus stop at gunpoint.[129] Gacy drove Donnelly home with him, raped him, tortured him with various devices, and repeatedly dunked his head into a bathtub filled with water until he passed out, then revived him. Donnelly later testified at Gacy's trial that he was in such pain that he asked Gacy to kill him to 'get it over with,'[130] to which Gacy replied: 'I'm getting round to it.' After several hours of assaulting and torturing the youth, Gacy drove Donnelly to his place of work, removed the handcuffs from the youth's wrists, and released him. Donnelly reported the assault and Gacy was questioned about it on January 6, 1978. Gacy admitted to having had 'slave-sex' with Donnelly, but insisted everything was consensual. The police believed him and no charges were filed."
"In March 1978, Gacy lured a 26-year-old named Jeffrey Rignall into his car. Upon entering the car, the young man was chloroformed and driven to the house on Summerdale, where he was raped, tortured with various instruments including lit candles, and repeatedly chloroformed into unconsciousness.[135] Rignall was then driven to Lincoln Park, where he was dumped, unconscious but alive. Eventually he managed to stagger to his girlfriend's apartment. Rignall was later informed the chloroform had permanently damaged his liver. Police were again informed of the assault, but did not investigate Gacy."
The short answer is institutionalized homophobia within the American police culture played a role. For decades, the American police's experience with the LGBT community was largely adversarial; it was to enforce anti-gay laws and in doing so the departments often inculcated a stereotype that gay men were degenerate and predatory.
Even post-Stonewall, this adversarial mentality continued, which led to an under-enforcement of the law to the LGBT community. The latter was often reluctant to engage with the police and the police and district attorney's offices frequently did not want to pursue charges. Some of this reluctance was structural (domestic laws were designed for heterosexual couples with a silent assumption that the victim was female and transgressor male), but the culture of homophobia certainly played a role. [Note, I am violating the 20 year rule here, but it illustrates a larger historical point] One 2005 study of Texas police officers found that not only was homophobic sentiments, but also there was a strong correlation between homophobia and a willingness to judge a gay suspect guilty. Such a mentality is deeply engrained within law enforcement and I would argue was even more present during the 1970s. There were no substantial institutional checks against homophobia within law enforcement during this time (such programs would only really come about in the 1990s and contemporary law enforcement discourse still laments their anemic nature). Although it's slightly counter-intuitive, such a mentality contributes to the propagation of "out of sight, out of mind" attitude which dismisses crimes against LGBTs because they are outsiders and not a group that needed protection.
The Gacy-Donnelly case was not the only example of police writing off or being disinclined to investigate crimes involving any hint of same-sex contact. Konerak Sinthasomphone was 14 year old Laotian boy who briefly escaped Jeffrey Dahmer. Neighbors called police about a naked boy, but believed Dahmer's story about a lover's quarrel. The transcripts on the police radio afterwards included jokes and homophobic remarks. After Dahmer was caught, the officers claimed that they were the victims of a skilled con man, which might make sense if one ignores the mountains of evidence against Dahmer (they were unable to communicate with Sinthasomphone, Dahmer first had him in a headlock when they came upon him, two black neighbors attested that this Sinthasomphone was not 18 as Dahmer asserted, Dahmer's own police record as a sex abuser, etc.).
Sources
Kwan, Peter. "INTERSECTIONS OF RACE, ETHNICITY, CLASS, GENDER & SEXUAL ORIENTATION: Jeffrey Dahmer and the Cosynthesis of Categories." Hastings Law Journal 48 (1997): 1257-1387.
Lyons, P. M., Christine M. Anthony, Karen M. Davis, Krissie Fernandez, A. Torres, and D. Marcus. "Police judgments of culpability and homophobia." Applied Psychology in Criminal Justice 1, no. 1 (2005): 1-14.
Spindelman, Marc. "Surviving Lawrence v. Texas." Michigan Law Review (2004): 1615-1667.