I'm not really sure the issue is whether or not females should be at a stag night. Personally, my interests are more tomboy-ish, and I'd be quite happy to do paintball, go-karting, etc. Much happier than I would be doing the stereotypical hen night activities.
I think a mixed party would be great fun. But the point in the OP is that it was planned as a guys-only event. This woman then invited herself along. Someone upthread said it's telling that all the guys agreed, but really, that sounds pretty typical for a group of guys. If she's particularly overbearing, they'd just say yeah sure, or whatever, and complain later on.
If someone were planning a girls' night (lets not call it a hen night, just say it's a bunch of girlfriends who want to go out, see a film, have drinks, whatever) and one of the girls had a bf who insisted on coming along, I think people would have an opinion on that. Either the BF would sound controlling (possible) overly lonely (possible) or just can't bear having a night alone without his gf (a wee bit sad). And yes, having the guy along would change the dynamic of the evening. There's jokes that might normally be made that wouldn't be with the guy along, or comments about partners, etc.
I should stress again (because the way this thread is going, I can tell I'll be accused of this) that I love my male friends, and I much, much prefer mixed parties. If I'd had a hen do at all, it would have had all my friends, not just my female friends. But the fact is, the OP's DF was planning a boys night out, and this woman who wasn't even his friend invited herself AND another woman along. WTAF, seriously. And the fact that she is making a huge deal and calling the OP a bitch tells me that she's an attention-seeker and other words I won't use.
Perhaps the OP shouldn't have made a big deal out of it, but it's done now, and I don't think this woman is a big loss from the hen night. 