The study of same-sex couples is a new field in demography. In France, as elsewhere, the census contains all the necessary data, but they are difficult to use because in many cases a miscoded sex of one of the partners results in a couple being identified as same-sex when this is not the case, or vice-versa. As heterosexual couples largely outnumber same-sex couples, "false" same-sex couples severely distort the observation of "real" ones. The French Family and Housing survey (Enquête Famille et Logements, INSEE, 2011), conducted in conjunction with the census, enables us to measure sex miscoding in the census. We observe that among the 139,700 couples identified as same-sex couples in the census, 58,900 turn out to be "false" due to a miscoded sex of one of the partners. Such errors concern 0.355% of heterosexual couples. The rate of error varies significantly by the respondent’s sex and the presence of children in the household. However, no significant difference is observed for other sociodemographic characteristics.