> Are there seriously investors that only back purely female teams? That seems ridiculously sexist and financially stupid to eliminate so many good startups that aren't all female.
That would neither be sexist nor would it necessarily be financially stupid.
As for the sexist part, reverse sexism is not a thing -- if the playing field is so imbalanced, then explicitly favoring the discriminated-against group is a fair measure to level the playing field. It's the same reason we have women's colleges and organizations devoted to advancement of women, and why similar institutions for men would be (generally speaking) incomprehensible.
As for the financial wisdom of investing purely in companies run by female co-founders, the whole point of investing is to find opportunities for investment that have been undervalued by the rest of the market. There are certainly some investors that have discovered that businesses run by women are undervalued in the market; maybe they have even calculated a rough figure for how undervalued they are, perhaps 15%. They may decide not to even look at male-led companies, as they would need to find 15% extra hidden value in order to match the hidden value -- unseen by the rest of the market -- they already know the female companies must (on average) have.