A question asking for respondents' sex is one of the standard sociodemographic characteristics collected in a survey. Until now, it typically consisted of a simple question (e.g., "Are you…?") with two answer categories ("male" and "female"). In 2019, Germany implemented the additional sex designation divers for intersex people. In survey methodology, this has led to an ongoing discussion how to include a third category in questionnaires. We investigate respondents' understanding of the third category, and whether introducing it affects data quality. Moreover, we investigate the understanding of the German term Geschlecht for sex and gender. To answer our research questions, we implemented different question wordings asking for sex/gender in a non-probability-based online panel in Germany and combined them with open-ended questions. Findings and implications for surveys are discussed.