Excellent point: That pushing a bit can make it self-sustaining.
If there's 5 women, among 300 men, any woman that -does- show up will easily feel like an outsider, as if she doesn't belong, as if it's a men-only arena.
Whereas if there's 70 among the same 300 men, it'll feel more like "there's more men, but obviously this is a both-sexes thing", which makes it a lot more tempting to show up, which can easily become a good spiral.