Probably the same thing that goes through the head of a guy when his wife pretends to be a sexy nurse or a high school cheerleader trying to get a good grade from the professor so she can still be on the team, whatever.
Roleplay is about fantasy. There are lots of fantasies that are "fucked up" in my view. I don't get the sc4t guys, I don't understand furries at all. But as long as everyone involved is ACTUALLY a consenting adult I don't give a shit if somebody is pretending not to be.
Same thing with CNC. I think r4pe is one of the worst crimes possible and am revolted by the idea of people who get off on that, but I get why both women and men have CNC fetishes and fantasies and re-enact them together.
I don't see why fantasizing about one crime is more acceptable than another.
Now I'm not a psychologist so I can't tell you if this is a healthy outlet for people who get off on it so they don't go out and commit real crimes or if it's a gateway for people who maybe had a tiny bit of it and it makes it worse and worse until they feel they NEED to escalate to actual crimes. I don't know what the research says.
So if there's data out there that says "we have multiple studies that show this is, in fact, extremely harmful" then I'd be open to changing my mind on it. But for now I see it as just another thing people do to get through how fucking unfair and terrible life is. You want a few seconds of dopamine from an orgasm and this helps you get there? Who gives a shit. You aren't hurting anybody, everybody's an adult, just nut and go about your day. Maybe it'll help you be nicer to the clerk at the store or the lady from the bank when she calls.