Sexuality is a fluid spectrum that is endlessly confusing. As such, it opens up a lot of room for misunderstandings. Fluid sexuality can mean that a person does not feel tied down to a specific sexual orientation and is instead open to change and new experiences, it can mean someone can redefine themselves and their sexuality over time without necessarily invalidating past experiences that don’t fit with the new label, or it can mean a hundred different things to a hundred different people. However, the general consensus is that sexuality has room to change and develop.
Now, many people (who more often than not tend to identify as heterosexual) accidently take this the wrong way. While, yes, sexuality is fluid and may change, it does not give you the right to invalidate the identity someone currently associates with. This invalidation may be unintentional, but it is harmful towards the people who it is directed at nonetheless.
Something invalidating could sound something like, “maybe you’re actually gay, not bi,” or, “you could actually be straight, sexuality is fluid after all.” You’re not being progressive, you’re making the person you’re speaking to uncomfortable.
If you don’t see the problem with those statements, then let me enlighten you. Someone who does not subscribe to the societally favored label of “heterosexual” didn’t come up with that realization overnight. Discovering and accepting the fact that you aren’t heterosexual is a long and scary process. If someone has made it to the point of coming out to you as their chosen label after spending hours and hours and hours mulling over it, then the last thing they want to hear is you telling them that maybe they aren’t actually gay or bi or pan or whatever label that person has chosen.
If you think about it, “you could end up realizing you’re actually straight” is just a roundabout way of saying, “it’s just a phase.”
While, yes, someone who previously identified as gay could realize they’re actually pansexual or vice versa, it’s not your job to make them aware of this fact. You need to respect whatever label someone has decided they’re comfortable with. In general, it’s always best to accept and respect what someone tells you about their own sexuality. They are the foremost expert on who they’re attracted to, and they always will be. You do not know someone’s sexuality better than they do, as I can guarantee you they’ve spent a hell of a lot more time thinking about than you have. All in all, don’t try to tell someone else what they do or could feel, and you’re in the clear.