It really depends what kind of baby you have, whether you can fit them into your lifestyle as easily or not. It's not *all* about the parents' attitude. I had one baby who I could have taken anywhere, and one who needed strict routine or she made life for everyone pretty hellish. Same parents, same attitude.
OP, I don't think you should have to change your whole group, or fit everything around this couple. However, something to bear in mind is that if you're all couples in your mid-twenties, it's fairly likely that more/many/most of you will be having kids in the next 5-10 years, and that the group might adjust more towards brunch in the park regardless. This might be the start of a trend, rather than one isolated couple, and it might be an idea to think about how you'll all react when it's two or three couples with kids, rather than just one.
And it's great to have all the best intentions in the world of being the kind of parent who just gets a babysitter and goes on with their life (me), but then you get a baby who screams blue murder if she's out past a certain time, and at 8 months, decides she just won't go to a babysitter, without all the screaming. It's hard work, and a bit of adjustment from my friends so I could see them occasionally was really greatly appreciated.