It's not that your friends are nuts, it's just that sometimes people WITH OR WITHOUT CHILDREN (it's not just the childless couples i have seen act like this) It's that they are unusually attatched.
i'm a dog lover, i have a dog. We spoil her. We are also a childless couple. And we treat her like our 'child.' even though i understand she is nothing like actually raising a child.. we don't want kids for a reason
However, i understand when people are a 'pet free' home and we just don't take her with us when this kind of situation arises. You friends should be more understanding, i would never assume to bring my dog unless i had specifically asked... so although our dog is like our child, i understand that the fact that she is a pet puts her in a different criteria than an actual child.
Your friends sound very attatched to their dog and i understand that semtiment, however, just gently tell them it's nothing against their "inset dog's name here," it's just that you feel it's not appropriate for them to bring their dog.
Sweetpea