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

People often don't realize that sexuality, gender, and romanticism are always fluid and ever-changing. When people are unfamiliar with a term (something like pansexual or aromantic, relatively new terms) they often take it upon themselves to decide what's easiest for them to handle. Denial is the easiest way to repress something. However, if they do truly care about you, you need to inform them about how their invalidation makes you feel and ask them to change how they treat you/which pronouns to use/et cetera.

Because people are off-set by differences, and don't feel comfortable around something that is different from their views. Just because you're different, doesn't mean your wrong. And embracing that makes you brave. Just because they think you wrong, doesn't make you wrong.

because everyone else is nosey, you do what you want to do, dont let anyone invade your sexual identity its yours for a reason

Some people have been raised to believe that only some realities are valid. Some are simply too close-minded, others might understand if given the chance to learn more about other realities. Dialogue can be very important, some people are not intentionally mean but simply ignorant about the LGBT reality. While some will not change their mind, other could if given the chance to learn more, spreading awareness is very important, although I perfectly understand that it can be frustrating sometimes when people won't listen. But if there's a chance to braoden someone's horizon it's worth trying.

I think that some people were just raised with a certain mindset. some people were raised to think that, for example, that being homosexual is "wrong". They just haven't been shown\educated on these subjects and were told that something is absolute when in reality nothing is absolute. Instead of thinking on subjects as black and white, think on them as blended colours.

When people don't understand something, they feel the need to fight against it, or even disregard it

Because they have a particular mindset about sexuality and when yours don't match with theirs..they condemn it..they do not understand your situation so it seems wrong or weird to them...its their wrong mindset

People feel the need to invalidate you because they are not able to fully grasp the human sexuality spectrum for what it is, a fluid river of love and harmony.

People may feel the need to share their opinion when not needed, if you act a certain way or do things that convey to a certain sexual identity they will point it out in jokingly matter or not, if you feel offended by that then you should confront them on that topic or simply state that you wouldn't like to be labeled as such and correct them

Usually, it is because they are insecure. For instance, if someone says that a person's identity is invalid, it is possibly because they are coming to terms with their own identity. It is also likely that they have been raised a certain way, and as people don't like change, they do not want to adapt to just anyone. In my experience, patience almost always wins out. Express what you are, and if they don't like it, wait awhile and above all, don't change who YOU are to appease someone else.