[bananashazama]

To what Japanese phrases are you referring? Most of the various phrases I'd expect to use to refer to a group (minasan, [name]-san-tachi) are gendered at all in typical use, and the ones I can think of that are gender only one member of the group (e.g. kanojo-tachi to refer to a group including one known female).

And it's not like Japanese doesn't have similar sexist norms baked into its own vocabulary either (if anything, to a rather greater extent than English). No, an everyday modern speaker probably doesn't mean anything by the fact that it's danna-sama but oyome-san, or the fact that formal speech (as one would use with a superior) is read as feminine-coded in casual contexts. But that doesn't mean there's not something behind those norms, either.

Whether the lecture is counterproductive is an entirely different question to whether the usage is sexist.