This research created an instrument to assess sexual fluidity-changes over time in attractions and sexual orientation identity and has examined whether experiencing sexual fluidity is related to specific cognitions, attitudes and beliefs among sexually and non-sexually fluid individuals. Participants (435 Spanish young-adult students, 310 females and 125 males) completed an online form which assessed demographic data, sexual orientation, sexual fluidity, and attitudes towards male and female bisexuality. An empirical sexual-fluidity factor has been isolated: women scored higher than men (d = .68) and bisexual people higher than homosexual (d = 1.74) and heterosexual (d = 1.40) groups. There were gender, sexual orientation and sexual fluidity differences in attitudes towards male and female bisexuality. Women considered male and female bisexuality more stable than men (d = .29 & d = .34); sexual-fluid people showed higher positive attitudes towards male and female bisexuality (d ranged .29 - .51) than the non-sexually fluid group; bisexual individuals scored higher in male and female bisexuality attitudes than heterosexual individuals. This study confirms previous researches which showed that females were more likely than male to have fluid sexuality. Attitudes about bisexuality are not a simple question; they are affected by gender, sexual orientation and sexual fluidity factors.