Recent studies have examined the different criteria a person could use to affirm his sexual identity. Using in-depth interviews conducted with Brazilian and American men, this study examines specifically the relationship between sexual behavior and sexual identity. The results indicate that being the passive person in anal intercourse is considered the final line drawn between being straight and being gay for Brazilians, whereas this sexual act is not necessary the determinant boundary for American men. Definitional criteria of sexual identification and of the sexual act boundaries are situational. Sexual identities are fluid in part because the criteria used to determine them are also fluid.