@bhmomma - in our part of the country it’s often the custom to hold “open-church” weddings, where pretty much any one who wants to can come to the ceremony. I didn’t do it and it appears to be dying out but it was quite common when I was young.
Yes, our niece had so many weddings to attend, as did her fiancé. They both chose to only attend the weddings where they were in the wedding party and generally didn’t attend weddings that the other was in, UNLESS they were both very close friends of the bridal couple. It would otherwise really add up in travel costs for them. Both were fine with attending without the other; both have a hard time getting away from work PLUS the travel costs just made it impractical.
@mom23travelers - I also had a cousin who was adamant about not having children at her black-tie wedding. The cousins with children pretty much boycotted it. I know we’re a minority, but in our family, excluding children simply isn’t done. My folks went and said the whole thing was boring and pretentious, ant to top it off they divorced less than two years later.
I love kids in and at weddings. Some of my fondest memories of my own youth were attending my older cousins’ weddings. But, I’m for weddings and other events that are family-centric with less concern about making them an instagram worthy moment. Good food, good music, family & friends = good party in my book.
It broke my heart to have to exclude the children on Mr R’s side. I love kids at weddings- they’re generally the ones having the most fun. They almost NEVER have a problem getting on the dance floor lol. We just couldn’t afford another 30+ guests (and no, that’s not an exaggeration) 
@doschicos - Agreed. And IMO, a good party doesn’t necessarily have to include frou-frou food or a museum-quality venue, either. My wedding reception was at our parish hall. We served heavy hors d’oeuvres,beer and wine and had a DJ. We invited kids and plus-ones and everyone had a great time. Having a wedding where one has to stress over the cost of each and every guest doesn’t sound like much fun to me.
I am at my nephew’s wedding (long weekend) now. I was having a pool side chat with a woman (friend of my sister), she said she was here with her husband and son. The son has been friends with my nephew since 9, but they have a daughter and she was not invited. The woman didn’t think it was strange or had an issue with it. They flew to a destination kind of place for a wedding, staying at a fairly expensive place, but had no issue with the fact their daughter was not invited.
For S’s summer wedding they are doing +1 for all single guests over 16. They don’t expect every one to bring someone, but want to give the option, especially to the cousins. There will be a few younger children–DIL has a niece and 4 nephews.
We went to a wedding a couple of weeks ago. DD was invited because she knew the bride. DS was not invited. Several years ago the brother of this bride got married. We went…and DS was invited…because he was friend,y with the groom. No plus one for my kids in either case.