Shower thought: if Chaos Space Marines weren't such an integral part of the setting nobody would believe they exist

Like imagine if Chaos was depicted almost entirely as human cultists and mentions of Chaos Space Marines were about as rare as, say, Chaos Eldar or Tau Genestealers or whatever. Like, only mentioned five or so times. Everyone would be saying shit about how the concept doesn't make sense. Space Marines are indoctrinated killers who have been brainwashed since they were children. There's no way Chaos could tempt them. How can Nurgle tempt a man that no longer fears death? How can Slaanesh tempt an eunuch with no real desires for pleasure? Maybe you'd get some guys saying that Khornate Space Marines make sense, but beyond that people would just laugh at the concept.

EDIT: I think a few people are missing my point. My point isn't that no-one in-universe would believe in Chaos Marines, or that its impossible that Chaos Marines should even exist. Its that the fandom would reject that as a possibility if they weren't around from the start, because everyone is kinda used to the idea that there are "no crossovers" and all the justifications as to why. And yes, I know Slaanesh isn't just sex, that wasn't the point I was making.