If I understood right, you are under the impression that the original post is from an incel calling out someone for being enthusiastic about determinist philosophy; and wrongfully likening it to a fetish. A few bits of context seem to have eluded you:

• The poster in the image has the lesbian flag on their profile pic. It may not be impossible for a lesbian to be an incel, but at the very least it takes Tumblr OP a bit further from suspicion.
• The screenshot is from Tumblr. You wouldn't be blamed for not knowing this, but some Tumblr users have a way of posting that consists of simply describing a new and humorous character concept that they came up with. This could be related to the fanfiction community. It is not a description or a critique of an actual person, just Tumblr OP thinking "wouldn't it be funny if...".
• Another thing with this very same Tumblr community is how common it is for them to talk and meme about kinks. Something Lemmy OP has picked on, judging by the title, is that the reason the fictional character is horny is that the absence of free will is similar to bondage or a similar form of erotic domination. This is the original joke.