B.L.
It can be difficult to be in different seasons of life with "friends". They don't have children, so they have no idea how difficult it can be to find a babysitter, especially at the last minute. I doubt that they are deliberately trying to exclude you by choosing non-family sorts of things to do. It probably wouldn't occur to them to go to the circus or the zoo or the children's museum. There wouldn't be much in it for them. Perhaps they don't care for children; they have that right, but it certainly doesn't make for easy get togethers with you.
I highly recommend Dr Laura's books on husbands and marriage (Laura Schlessinger). Feminists hate her (and hate men), but her books explain the differences between men and women, and what men need. They need to be #1 to their wives, and admired and praised and thanked for what they do, just as women need to be wanted and appreciated by their husbands. Your relationship should come before your relationship with the children (studies show that that makes for more security and happiness in the home). It helps to plan ahead for date nights, and have sitters lined up if needed. After reading The Proper Care and Feeding of Husbands, I made a few simple changes (stopped nagging, stopped pouting, got off his back about his hobbies, started welcoming him home with a smile/hug instead of with anger that he was late or whatever) and our marriage is now fantastic. I'm no longer hurt and hostile most of the time, and am deliriously happy with our marriage (unlike before when I used to throw around the D word and was forever pissed off). We have a heated discussion about 6 times a year (or less) instead of that many times a week or day. He looks forward to coming home now, and spending time with me, instead of dreading it. Perhaps if your hubby was more fulfilled at home, he wouldn't be looking to maintain ties with people he no longer has much in common with. I'm not trying to insult you at all; I've been there. I can't recommend the books enough. I'm reading the Proper Care and Feeding of Marriage now. Of course, maybe he hasn't grown up yet for whatever reason (still trying to have a perfect childhood?) Dr Laura has another book that's still sitting on my shelf (I'll get to it...) called Ten Stupid Things Couples Do To Mess up Their Relationships. It is probably worthwhile reading as well. I don't think I would say any more to him about severing ties with these "friends". Let him come to that conclusion himself.
It helps to have friends with similar children to your own (another couple with 2 kids about your kids' ages) that you can trade babysitting with. People are often happy to do it when that means you'll reciprocate. Parents of young children are often home for dinner and all that anyway, and might not mind being called upon at the last minute, again if it means that they can do the same with you on occasion. Good luck!