According to the linked article the survey was limited to heterosexual individuals.
Even if homosexuals were included in that survey, and asked about "how many heterosexual partners you had", them answering "0" wouldn't matter at all (provided that there were roughly the same number of gays and lesbians surveyed)
Source? Or is this just guesswork? Because the stats I've seen in the past, while varying in numbers, have pretty consistently been that, in the population, from most to least common is:
Opposite sex partners only > Both same and opposite sex partners > Same sex partners only. (Don't have any sources handy, but I'll try to find and post some later.)