While the demography of sexuality has become increasingly important, demographic methods to estimate the proportion of sexual minorities are still in development. Previous studies in demography tend to assume that sexual minorities can be clearly distinguished from the majority. This paper focuses on a response category—“don’t want to decide, haven’t decided”—to a sexual orientation question used in population-based surveys in Japan, in which the issue of heterosexual respondents being misclassified as non-heterosexual and the difficulty of fully separating heterosexual and non-heterosexual people surface. The findings from an experimental web survey conducted in Japan indicate that over 20 percent of those who selected this category did so mistakenly or because they did not understand the question. The majority of those who selected this category, who may be classified as queer/questioning, did so for a variety of reasons. All this illustrates the complex nature of and the difficulty of measuring sexual orientation.