Why do others feel the need to invalidate my sexual/gender/romantic identity?

Simply because people don't understand. People fear and discredit what they cannot comprehend. Because they can't wrap their head around the whole concept they just discredit it not stopping to consider that it might just be something they are incapable of understanding.

Because they're uncertain about their own identity, is my guess! Maybe you have something that they want, but are too afraid to go after themselves (even if it is your bravery to live your life as you think is best for you). Others may feel threatened due to 1000 reasons (none of which are acceptable). You can try and re-assure them that you are not a threat, but if you have to do this so much, you may wish to consider if this relationship is worth keeping. Finally, people like parents may be worried about the life you're going to have (if any of these things is atypical), and they may feel like you won't be happy (although I am sure you know otherwise). If they invalidate your identity, they could just be trying to protect you (in a very weird way). However, with time, many people like this see that you're actually happy, and realize there was no need to expect you to be someone else. :) Good luck, my friend. We're here for you.

Those that don't understand often feel the need to invalidate information that doesn't match up with their perception of someone else. It's not you personally, it's their world-view and the lens in which they view things. It's not you, what you think, or what you feel that's invalid as emotions are valid, normal things. It's the other person that needs to work on the way they perceive life and people around them.

Because people often feel strongly about their beliefs and try to force their beliefs upon others because it makes them uncomfortable having someone going against their religion/beliefs. Some may not know who they are and it could feel easier for them to put others down or say they are wrong or don't know what they are talking about

I get a lot of questions about people outside of the LGBT community treat us so bad or try to tell us things that we know aren't true. But, I don't know. I ask myself this everyday. Some people just don't understand, some people don't want to. The only thing we can do is know that no matter what they say, who we are is completely valid.

Maybe their just busy bodies who want to see peoples life because their life is boring. Don't take it too hard just take it as they find your life more interesting.

Often times it is their own insecurities about themselves. They tend to be jealous, because you are able to identify yourself and they are not. Unhappy people also feel the need to bring others down since they themselves are miserable. Remember who you are and do not let other people worry about your identity for you.

People are born with frames of thoughts. thoughts that don't fit in their frame, they invalidate it so as to not cause incongruency in their own cognitive thinking. Also, society is often reluctant to accept differences that don't fit in the norm. People are afraid to refute the majority common sense so once they feel threaten and don't understand something. They become afraid of it. When they're afraid of something they don't understand, they lash out at it and all to protect their own sanity. There are multiple reasons why people refute certain things so it's best not to ask why people invalidate you but ask why does it bother you that people invalidated you.