Who cares? -er suffix makes more sense in terms of how we make nouns out of verbs. -ist is rarer. Either work for me even if -ist is the sexy popular one.
A lot of these posters have wrong messages. Rape IS sex. Sexual assault is involuntary sex. Sex can’t be divided from sexual assault, it’s what defines it apart from other forms of assault. I do agree with the ‘just because you help her home doesn’t mean you help yourself’ poster though.