fluidity vs bisexuality

The most important difference between the two is that a bi person has a stable sexual orientation (is attracted to more than one gender), while a fluid person tends to be sexually attracted to one gender but, in certain contexts, may be sexually attracted to another. The emphasis of the difference is on stability versus change. In more technical terms, sexual fluidity is a predisposition to sexual flexibility that allows us, at some point, or from some point onwards, to sexually desire people of a gender that normally doesn't attract us or perhaps hasn't ever attracted us (or ever will again) (Diamond, 2016). In other words, sexual fluidity implies that sexual orientation is changeable and that this change may only be sporadic. However, it is not always easy to distinguish sporadic or fluid experiences from bisexuality. Nor does it mean that, from a research point of view, variations in sexual orientation are fully understood - in this respect we are still in need of research insights!

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Diamond, L.M., Sexual Fluidity in Male and Females. Current Sexual Health Reports, 2016. 8(4): p. 249-256.