I can't
know that we do anything wrong, but the (felt, not measured) numbers feel like they are too large to be explained by purely biological reasons. (Yes, is this by no means scientific. And that bothers me; but I just don't know any numbers).
Also the articles I linked to here mention things that deter some women which I've observed in Perl community, for example endless discussions that sometimes end in insults (seen on p5p and on some IRC channels) and the very competitive nature of XP here on perlmonks.
I'm not saying that we're doing anything wrong a priori, but assuming that we do everything right because we haven't measured that we're doing anything wrong is too easy, IMHO.