Even if the probability of receiving a micro aggression is equal for both men and women, only those which are towards women and seem to be caused by their sex will be elevated to the level of explicit political discourse.

Consider the D&D example given in this post. The DM saying "no, you're playing my game wrong" is easy to interpret as a micro aggression, but to gamers (especially ones who've sat at both sides of the table) it's seen as part of gaming, and someone who gets upset about it probably shouldn't be at the table (in part because they can probably find a DM more suited to their interests). This particular example is being discussed publicly because a poster thought it was an example of sexism; if someone had posted a similar anecdote on the site outside of the context of LW Women it would not be seen as anywhere near as relevant.