There were a lot of interesting replies giving further detail. I'd sum it up by saying that women say they are more likely to experience reply guys when
* posts are boosted widely
* they're more visibly female
* they're talking about tech
* responses come from certain servers, including fosstodon and mastodon.social
A number of women mentioned discomfort seeing reply guys on other women's posts. When I worked on abuse previously, this sort of bystander harm had a significant chilling effect on the willingness of people to post when they identified with the people targeted.
To me, this is a pretty clear example of what Kate Manne describes in Down Girl: "misogyny enforces patriarchy by punishing women who deviate from patriarchy".