In my 20s, I dated men exclusively. I had a few short flings with women, but they never really turned into relationships. After a few years with my last long-term partner, I thought I was mostly heterosexual. But then, when I joined my local kink community, this label didn't fit anymore: I wavered between "heteroflexible" and "pansexual", only to go back to my original teenage orientation: bisexual.
From Undecided to Scientifically Proven Sexual Fluidity
Twenty years ago, this fluidity would have been interpreted as "undecided", "it's just a phase", or "closeted lesbian". (A lot of this stigma is still applied to bisexuals, by the way, but that's another topic.) However, research in sexual fluidity published in the last 10 years or so has changed the way sex scientists, at least, consider sexual orientation.
This discovery, however, causes problems. If gay rights are based on the fact that we can't choose our orientation, then the possibility of fluidity can work against them. To solve this issue, scientists separate two things: sexual orientation and sexual orientation identity. Sexual orientation is generally stable across a person's life, mostly because scientists believe it is biological in origin. Yet sexual orientation identity, or how we name our sexual desires, attractions, and behaviors, is socially determined. That makes it much more flexible.
As my own experience shows, sexual orientation identity can, and often does, change, especially for women. The fact that I came back to my original orientation doesn't mean that the changes were meaningless. In fact, with each change of identity, I acted in ways that heterosexual, pansexual, and bisexual people are "supposed" to behave. My innate orientation, or who I'm attracted to, never changed; but how I approached relationships with people of different genders definitely did.
The term "sexual fluidity" is generally attributed to developmental psychologist Lisa M. Diamond. A professor of psychology and gender studies at the University of Utah, she published a very influential book in 2009: "Sexual Fluidity: Understanding Women's Love and Desire." This book followed a group of women over 10 years. Diamond contacted them at regular intervals and asked them about their relationships, sexual orientation and sexual identity. She discovered that many of her participants changed their orientation identity throughout the study, even though their attraction didn't change as much.
Other studies have suggested that men have a much more stable orientation and orientation identity. However, women, apparently, are much more flexible. This effect can even be seen when you study people's physiological reactions to sexual content. Heterosexual men tend to react to men-women and women-women content only; heterosexual women, however, appeared to react in somewhat equal ways to all kinds of content, including non-human sexuality.
What does it all mean?
From a scientific standpoint, it means that we still don't understand human sexuality as well as we think. It's not just homosexual versus heterosexual: it's also everything in between, and how it can change over a person's lifetime.
From a more personal standpoint, it means that changing your mind about your sexual identity isn't a sign that something is wrong with you. In fact, enough people do it that it's become a subject of study in its own right. You're not confused, in a phase, or hiding in the closet: your sexuality is valid, real, and important.
In fact, the concept of fluid sexuality shows that nothing in this world is black and white. There are all kinds of gradations in between. When we are open to that, we also become open to a wide range of nuanced experiences, people and flavors.