People feel compelled to weight in.
Look at the numerous bathroom laws against trans people not being able to legally use the bathroom of the gender they identify with (the gender they are). It boils down to someone else thinking they have the right to judge an individual's gender identity.
People are furious and angry at people who don't identify with the orientation consistent with their behavior. (>>>referring to a site to hook up "straight men" with gay man)
1) The individual knows best.
2) It doesn't matter if the orientation isn't correct by your definition or by the world's definition.
3) It's not fair... doesn't have to be. (S/he is acting X but doesn't identify as X so gets none of the negatives of being X... I might point out & none of the positives.)
4) We don't know who is questioning or seeking answers through testing label identity. (S/he might do X to prove to themselves or the world they are X and they might be X or they might prove to themselves they are Z.)
5) It doesn't make sense to us. (Doesn't have to)
I've made the mistake of trying to process a friend's orientation with them (because "it didn't make sense to me"). I wasn't doing it to be a terrible human being but I was struggling to make my version of a label fit.
Here's some advice: Just Listen. Process later when you're alone. Usually the person is sharing something very private and maybe very new to them... Just listen.
Person sharing the new information it's very brave of you to share something so personal... but some helpful hints:
1) Don't seek validation from the person you are telling. They might need to built a framework to insert the information you've given them.
2) You have nothing to prove and if you give examples of why you are X... and it doesn't fit their "definition or belief/myths" about the orientation/gender it creates confusion.
3) The reaction you get sometimes has more to do with the person reacting/processing then what you just told them. ((It's not all about you... other people have issues too.))
4) If the person is trying to be supportive cut them a bit of slack... This isn't a made for television movie so they may not have a script.
Orientation and Gender Identity is a personal matter... Just accept.