I just want to point out this is very much cultural:

• In my home country (Portugal) women were maybe half of the pupils taking IT Degrees and maybe half the programmers I worked with were women. (This is apparently also the case in many Eastern Europe countries).
• In The Netherlands, there were fewer women in IT but still quite a number of them.
• In Britain they were almost absent. Working as a freelancer (so in more places than a permanent employee would) in over a decade I've worked with maybe 3 female programmers (out of, I guess, 100 or so colleagues), all foreigners. In management, sure there were women, programming not so much.

Funnilly enough the most machist environment I worked in was in Britain and it actually had the highest proportion of women (none of which a programmer, most low-level management) but they were there due to gender quotas and were to a large extent treated as eye-candy by their own management, seemed to have been selected for looks and whilst some of the women seemed competent, others were not and were clearly playing that game.

I've seen how male-only environments tend to end up with this wierd disfunctional dick-wagging culture (though that's not at all guaranteed), but if there's one thing that specific case in Britain alongside my experience working in places were some of our colleagues were naturally women showed me, is that women need to be there on merit and thus accepted as equally capable colleagues, otherwise the result can be a lot worse than a "mere" macho dick-wagging culture were wannabe "young-bulls" in their ignorance think that "women can't code".

PS: I was thinking about it some more and just wanted to add that the only environment I worked in The Netherlands with almost no women (this was a small Software Products company of maybe 30 people and if I remember it correctly there was a single female secretary) did NOT have a "bro" culture at all. I suspect this was to a large extent because most people there were out of their of their 20s, plus the manager-owners were pretty decent guys (plus the Dutch culture itself) so it was a mature adults environment not a bunch of young-bucks dick-wagging and posturing.

A thing often left unmentioned in all the talk of machismo on startups (in the US) is that it's a very ageist environment relying on young guys who are willing and capable of working 12h/day and that definitelly makes such companies tend towards the immature end of the spectrum.