Thinking that sexuality is hard-set and locked in place forever, and not understanding that a new outlook on your sexuality doesn't mean your relationship was a "waste of their time".

Always knew I was bisexual in high school, highly preferring women and found guys only slightly attractive. Fast forward to 3 years into my last relationship, I found myself no longer being attracted to my then girlfriend, and took some time to myself to do some "soul searching". Came out finding guys to be more my speed. Everyone else I told was supportive, but my girlfriend was disgusted and said the whole three years we spent together was a waste of time, regardless if it was both of our first relationships and one to learn from immensely, not throw out and consider it a waste.