Obviously, in the scenario you describe, people will continue describe themselves as "anti-rapist" and everybody will understand that they mean that they are opposed to rape.

There is no "loose-knit ideology/movement" called "antifa" - there are groups like SDS and Don't Shoot PDX and a zillion others who describe themselves as "antifa", using it as an adjective. I'm aware of no person or organization who has attempted to proclaim that they are the one true antifa org.