The statistics justifying this particular prejudice are common, but the most popular seems to be from the "classic" 1978 Bell and Weinberg study, which found that:
Of 685 homosexual men, 589 (83%) had 50+ partners in their lifetime, 497 (73%) had 100+, 394 (58%) had 250+, 284 (41%) had 500+, 182 exceeded 1000 partners, an astonishing 26%. And 79% noted that over half their sexual contacts were total strangers.
However, this data is not really accurate of the gay population as a whole, since (a) the homosexual sample used was not random, but taken from people found in gay bars, personal referrals, gay organizations, public advertising, and mailing lists, and (b) the heterosexual sample (which was random) was not given by comparison to provide a control group. (Jeramy Townsley, http://www.bridges-across.org/ba/tidh/jones_yarhouse.htm)
A later study by Laumann in 1994 discovered that "Homosexuals still have 3-4 times as many partners as heterosexuals." However, as Jeramy Townsley also pointed out:
...what Laumann fails to explore is the radically skewed nature of the data. Typically this indicates that the mean, the statistic presented by Laumann, may not be the best measure to report. A further analysis of the GSS data (on which Laumann based his results) indicates that the median (50th percentile) number of sexual partners for heterosexuals is five and for homosexuals is six (http://php.iupui.edu/~jtownsle/gss/partners.html). The discrepancy between the mean and median is indicative of a small sub-population of gay males who tend towards high rates of sexual partners, skewing the mean, while the majority of gay men tend to have rates about the same as heterosexual males.
In other words: yes, there is a sizable subculture of homosexual men who are extremely promiscuous. The odds are, though, that the gay guy living on the next floor isn't one of them.