The study does address the possibility that gay men could be more likely to lie about their size and determines that they are not more likely. But.... they give no sound argument as to how they determined this! Intuitively, I am confident that gay men would be more likely to lie. There are far more gay "size queen" men than there are hetero female "size queens." Hell, the term, "size queen" itself originated in the gay community and was appropriated later by mainstream society. Gays certainly have more incentive to exaggerate.
Personally, I have had far more gay men comment on my size, check out my bulge etc, than I have women.