It certainly sounds like it's a problem with the friends, not so much the invite. While some childless people realise that kids make it a package deal, some people really aren't 'kid' people. The dynamic in my social circle has changed dramatically over the past 12-18 months as we've gone from 1 couple with kids and the rest either couple without kids or singles, to 4 couples with kids (me and DF will make 5). We used to often have house parties that went until the wee hours of the morning and would invite our friends with kids (including the kids) and would keep it fairly family friendly until they went home so the kids could go to bed which is when we'd start boozing it up a bit more. Now it's dinner dates and we're all home in bed by 10 and we don't see so much of the singles or the couple who aren't big on kids. I think it's normal for friends to change as life changes. We aren't all friends with the people that were our BFF's in high school because life has taken us down different paths. It's the same with when we decide to settle down and start a family. People either grow with us, or away from us.

I'd make sure you communicate these issues with your DP assuming you already haven't so he knows why you're not happy about it. Perhaps suggest rather than flat out declining an invite to go out, say to the sports bar to watch UFC, he can say "LMS could go into labour at any moment so I don't really want to go out, but why don't you come here instead".