The “friendzone” only exists when you end up in the mental gap between “I want to have sex with this person, but don’t care about them as a person” and “I want to have sex with this person, therefore I must care about them as a person.”

I think there's definitely some confusion and frustration that comes from people who see their peers hooking up, but can't figure out what transitions them from a friendship to a romantic relationship. Add to this, a certain low-key distrust cultivated by social media, wherein one person may assume they are getting strung along while the other isn't willing/able to clearly signal their intentions.

Normal consenting adults can meet someone, vibe, and engage in sexual behavior without necessarily more.

Well... sometimes. The term "getting lucky" is apt, as there are so many variables - some totally beyond either of the participants' control - that can determine whether or not the magic happens.

Once you understand that sexual desire is not romantic desire, and that romantic desire is actually really unfulfilling if not reciprocated, you’re usually good.

No. You're "good" when you've found a person for whom you can reciprocate romantic love. Just understanding the difference isn't fulfilling so much as it is enlightening. But it's like hanging a steak over the head of a hungry dog. One dog heedlessly leaping at it isn't more or less fulfilled than another who has come to the realization it is forever out of reach.