Seymour,
Your friends need to grow some nuts and prioritize their friends into their lives. It will never be like it was when you were single, and you need to understand that (which I think you do), but they also need to make an effort. When you start dating someone, and certainly when you get married, priorities and responsibilities change. They change further when you have kids.
Having said that, IMO people need to maintain their individual lives, separate time and friends. I love my wife, I love my kids, I love spending time with all of them and wouldn't want my life to be without them, but I still make time for my friends and do shit without my family. My wife does the same.
We have plenty of responsibilities and time that has to be dedicated and focused to our family, but there is still time for friends and personal time. Between play groups, birthday parties, dance practice, soccer and all the other shit that comes along with having a child, time is limited. Child care is another issue for a lot of people with kids.
Frankly, they need to make an effort and you should tell them that. I don't see my friends as much as I used to, or would like to, but I still make an effort and do see them regularly. You also need to understand they have other priorities and responsibilities that will take precedent (and let them know that), but that god f*ing damn it, they should grow some stones and hang out with you periodically.