Since OP mentioned that the company in question is a startup which implies a fair chance that they not yet have a HR department or that even some of the founder

Since OP mentioned that the company in question is a startup which implies a fair chance that they not yet have a HR department or that even some of the founders took part in this sexist behavior, IMHO OP should not hesitate to call out names (only the company's) as it seems it is the only way to punish this behavior.