Not really. There are groups that are men's only and nobody bats an eye about it. (Sport teams, dating/relationship support groups). They just have to be something unique to men.
If there is some functional reason why a group might be all one sex, then few people would argue against it. For example, the way men act, think, are expected to behave, etc, in a dating/relationship setting is different from how women do; and importantly the behaviours that lead to success differ depending on whether you are male or femaile. So in that setting it'd be entirely appropriate to have a all-male or all-female support group.
Is being a startup founder like that? I don't know, but it seems plausible that it might be.
