Part of the trouble, I think, is that there’s this pervasive myth that, not only is it appropriate for men to approach women and try to initiate flirtation in random places such as buses, but that that is the only appropriate way for men to meet women. I’ve read it specifically pointed out that this is not true, but I’m afraid it’s easy for me to forget. Most of the posters in the xkcd forum thread on this comic, for instance — including some of those who were actually criticizing sexist comments by others — apparently accepted that the only way for people to meet is in encounters between strangers, without introduction by mutual acquaintances.