Comment by daenney

To be perfectly honest, as a man, I don't feel comfortable being in rooms full of people I don't know regardless of gender but I feel more uneasy when it's only men. Somehow it feels more threatening to me. Being regularly beat up by a group of boys in school probably didn't help. Some of those feelings stick around.

With regards to the rest of your question; I don't think most men are presumed to be rapists and I don't think that is what the author means. There are also many other forms or perceived threats of violence that can make people uncomfortable.

There are multiple studies on the topic of male/female group dynamics that you might find interesting to read on this topic.