Yeah I had to go back and read that line again. I like Zizek a lot but he could also be a cis philosopher that doesn't understand trans identity and if his Lacanian psychoanalytic terminology hurts more than helps, just throw it in the trash. I tied that back to his previous line about Marx:

So, in the same way in which, for Marx, if one wants to understand the “normal” functioning of capitalism, one should take as a starting point economic crises, if one wants to analyze “normal” heterosexuality, one should begin with the anxieties that explode in transgender people.

Bourgeoisie economists thought of capitalism as functioning normally during the "boom", but they thought of the sock market "bust" as an aberration, unrelated to the normal functioning of capitalism. In fact, the boom and bust are two sides of the same coin. During the boom, profits are soaring because capitalists think they can make more and more money with worse and worse investments until the snap back to reality. Marx was the first to show that the crash was also a constitutive part of capitalism. If you want to understand a system fully, you also have to understand the exceptions to the rules. To return to the matter at hand, when we talk about the way that our society fails trans people, it reveals the way that our way of thinking about sexuality as a whole is a failure, but it is easier to ignore that in heteronormative culture. It only becomes apparent at the point of crisis.

my point is simply that the universal fluidification of sexual identities unavoidably reaches its apogee in the cancellation of sex as such. In the same way as, for Marx, the only way to be a royalist in general is to be a republican, the only way to be sexualized in general is to be asexual.

The Marx reference is, I believe, a reference to the 18th Brumaire. Monarchists (who opposed voting and elections) turned out and voted for Napoleon III (to overthrow democracy). If they were true believers in monarchy, they would not have voted, and Napoleon wouldn't have won. Zizek is always interested in these sorts of contradictions. Young adults today are given more "permission" from society to engage in sexual freedom than any other time in history, but they are having less sex than their parents. He sees an anxiety that cannot be put it into words.

Zizek is adamant that sexual anxiety is never going away, and that it will always be used as cannon fodder for the reactionaries. Part of the reason sexual anxiety will never go away is because sexual desire comes from the pre-linguistic, lizard part of your brain and so it can't be expressed in definitions, language, or binaries like male and female. We cannot understand our sexuality. All attempts to understand it are pathological. It is pathological for conservatives who project their sexual anxieties on trans people. Unfortunately, it is also pathological for leftists who want to help as well.

We should do everything in our power to help our trans comrades, but in 15 years there will be another crisis of sexual anxiety. That's my take at least.