Intimate partner violence is a recognized public health and development issue that is consistently and comparatively measured through women's experience of physical and/or sexual acts by their partner. While physical intimate partner violence is covered by a wide range of behaviors, sexual intimate partner violence (SIPV) is often only measured through attempted or completed forced sex, ignoring less obvious forms of sexual intimate partner violence. We explored women's conceptualizations of SIPV by conducting in-depth interviews with 18 Tanzanian women. Using a thematic approach, we identified key features of women's sexual intimate relationships and their perceptions of them. The women clearly defined acts of positive sexual relationships that occurred with mutual consent and seduction and SIPV that included acts of forced sex and sex under the threat of violence. They also identified several acts that were crossing the line, whereby a discrepancy of views existed whether they constituted SIPV, such as having sex when out of the mood, sex being the duty of the wife, sex during the menses, requests for anal sex, having sex to not lose the husband, husband refusing sex and husband having other partners. Women in this study felt violated by a far wider range of sexual acts in their relationships. Future studies need to improve the measurement of sexual intimate partner violence to allow the collection of encompassing, yet comparable, data on this harmful phenomenon.