As I understand the "RadFem" position, it rejects gender (man/woman) as socially constructed and oppressive to those in the Woman role, but embraces sex (male/female) as real and seeks to create safe spaces for Females. Yet such a sex realism simply moves the gender essentialism, whereby gender is an inescapable consequence of "socialization" as informed by anatomy, to a further remove. Essentialism in sheep's clothing. The motivation behind the whole fruitless endeavor seems to be to exclude trans-women from female liberation movements.
RadFems hold that women who are male (by some ostensibly scientific standard, I guess, or by some less-than-scientific claims of "socialization") are members of an oppressive class. Which, on its face, is ridiculous in my view. Trans*-identifying people are one of the most marginalized and vulnerable segments of society, and any movement which considers them to be oppressive is highly suspect to me.