I've always believed "respect is earned, not given" to be utter BS, but that's even more true when it comes to how people identify. June is LGBT+ Pride Month, which means you're going to be hearing about a lot of different identities (gender- and orientation-wise) that you've probably never heard of.
Please, for the sake of everyone involved, don't be an ass if you don't understand what they identify as. At one point, everyone has questioned an identity that they came across (and if you say you haven't, I'm going to say you're lying). Do that in your head, but be respectful to the person.
I've been online for years, and I'm guilty of bashing people's identities because I thought they were "weird" and didn't fully understand them. Guess what? I recognize that as being a horrible thing to do and have since matured.
It costs you nothing to be respectful.
When I see an identity I don't fully understand, I either ask the person about it (respectfully) or shrug it off because it's none of my business. The most it affects me is when it comes to their preferred name and pronouns, but even that isn't a big deal. It won't end my life if I call someone by a set of pronouns I don't understand.
Now, I'm not saying to not ask questions out of fear of being disrespectful; I'm saying to not be a total jerk when asking.
When in doubt, ask them about it. "Hey, can you explain what ____ means?" is a very different way to start a conversation than "I've never heard of ____ and think it's gross/wrong, so it doesn't exist."
The worst possible thing you can do is tell someone their identity doesn't exist. That pretty much tells the person that they don't exist, which is really just a dick move.
Because, again, what does it cost you to be respectful?
That's right, nothing.
Their identity doesn't hurt you in any way. Them being gay or trans or somewhere in the middle or both literally does you no harm. Respecting them does you no harm.
You may not understand if someone identifies as a "non-binary pansexual they/them," but they know full well what it means. That's all that matters. All you have to do is respect them and call them what they want to be called rather than what you think they should be called.
Nobody knows someone better than they know themselves.