Op, are you sure you aren't muddling up "don't understand" and "don't approve""? I can't imagine that you genuinely can't get your head around the various reasons why people might choose a child-free wedding, ranging from budget, personal preference to bridezilla-ness. Is it not rather the case that you don't agree with child-free weddings?
I am a big believer in people having exactly the wedding they want, as long as they understand that their guests are just that - guests, not hostages to their big day, and that some people may choose, or be unable, to attend where there are restrictions on children attending, or where the wedding is somewhere very inconvenient. I have no time for people who arrange incredibly lavish, complicated events and then throw a hissy fit when someone declines the invitation.
Over the last decade or so I have been to all sorts of weddings. We went to two weddings in the same social circle, both of which had children in attendance and both of which were well-suited to children being there. Both were very nice, but unfortunately both ceremonies were considerably disrupted by the same child whose parents (friends of ours who we were sitting next to squirming with embarrassment) declined to remove him, even when he was at the altar yanking at the bride's dress - at the second wedding I finished up taking him out after a lot of glaring and muttering by the bride's grandparents. The third wedding in the same social group was child-free and I suspect the behaviour of these parents was a big deciding factor - it was also nice and at a convenient venue. Most people with children attended, a couple had to decline but no-one seemed too offended.
I have also been to a wedding with DS as a small baby - that was a "babes in arms only" event and at a venue where you could stay on site which made coming and going with DS easy. We had a pleasant enough time although I did miss most of the ceremony as I had to take DS out when he cried. The format and venue of that wedding would have made it hugely unsuitable for toddlers and I think the bride and groom were sensible.
On the flip side of the argument, I have been to a very elaborate wedding in a location that was about as inaccessible as it was possible to be. It involved a big time committment and a lot of expense and the fact that it was child-free turned it into a logistical nightmare for all involved. It caused a lot of bad feeling because the very unusual venue meant that children could easily have been accommodated with babysitters away from the actual wedding proceedings if a little flexibility had been involved. As it was we had to drive 100 miles to leave DS with family, stay over at theirs, get a plane and then a taxi, pay for a hotel and then do the whole thing in reverse with the babysitting relative taking a day off work to help out.
As long as both guests and bride and groom are aware that both sides have preferences and requirements, and are prepared to be understanding about difficulties, then I have no problem with child-free weddings.