Reading through this thread I've gone back & forth on how I feel about this. I'm big on girls only get togethers. I have several different groups of girls who get together for different purposes. And a male inserted into one of these events without the previous agreement of all the girls, and the option to bring their male SOs, would not go over well.
However, in this case, it's the hostess's birthday party. And as such, she has the right to invite whomever she pleases. So inviting the girls and her boyfriend as the only male, well I kind of give her a pass. It's her bday party after all. Saying stuff about catching up with the girls is probably not that great of an idea but she's probably just not thinking in those terms of "girls only". She's thinking in terms of "people I want to celebrate my bday with", most of whom happen to be girls, but also includes her new bf, especially since they're still in the "honeymoon" stage.
Now, as a guest, if I got the idea that it was actually girls only, which is easily assumed based on her comments, I'd be pretty surprised to see her boyfriend there. It would definitely be averse to my expectations of a girls' night out. So I'd have to adjust my expectations and possibly my conversation to better suit the atmosphere. I don't think I'd be miffed that my DH wasn't invited because, again, it's her birthday and she should invite those with whom she wants to celebrate.
Now, if you kick out the birthday aspect of this, that would completely change my answer.