I got this one a lot for being poly. I started being poly over a decade ago and when I told people they often acted like I was gonna grow out of it soon, or once I met a nice monogamous boy I liked enough.
Unconventional sex/relationship (and related) things seem especially prone to garnering this reaction. A certain type of person just can't conceive that anyone might really, truly be wired differently than them on matters said person finds deeply important.
Same. People told me that me being bi and poly-minded was just a phase, I was confused, grow out of it, etc... 25 years later and I'm still in that mindset.