The thing is, I sort of agree with you, OP.
In the narrow terms in which you phrased it, turning down a wedding invitation as a form of protest against the very concept that it's a child-free wedding and you disapprove in principle is a bit precious/silly (not sure it's "selfish", though).
However, turning down a wedding invitation because it doesn't include your children and you don't want to spend the weekend apart from them (even though you could get childcare if you wanted) is fine. Not everyone sees huge amounts of their children in the week. Not all families get lots of time together at weekends. And children are only young, and keen to spend time with their parents, for a fairly short time. Also, very few people have infinite free childcare available. Free childcare generally involves calling in favours and there's a limit to how much of it is on tap. Using it for a wedding means it's probably not available for another occasion.
For us, we'd attend a local/local-ish child-free wedding that didn't involve an overnight stay. Something further afield we'd probably turn down if it didn't include children.
We actually went to a wedding (with the children) this past weekend. In order for us to go, the children had to miss, between them, one birthday party, one bowling trip, one rugby tournament, one play performance (so that child couldn't be in the play at all), one play rehearsal (different child, different play), a swimming lesson and an art group. This was a reasonable trade-off, because they got to go to the wedding. If it had been a child-free wedding and DH and I had gone alone the children would still have had to miss all that stuff but they wouldn't have got anything out of the deal. In practice, these days, if it had been child-free then we just wouldn't have gone but, on the other hand, when the DCs were younger and didn't have their own lives we probably would but, on an imaginary third hand, when DH was working away a lot and only got one weekend with the DCs every month or two we probably wouldn't.
i.e. there are a whole host of factors at work for every family that lead them to make the best and most appropriate decision for their particular circumstances at that particular time. And setting yourself up as an arbiter of what decisions are "acceptable" or "unacceptable" is a bit wanky.
I still agree that if couples are genuinely turning down invitations to weddings that they would love to and could easily attend with no childcare problems at all while the children blissfully did something else (and the family has lots of free time together the rest of the time) that that's a bit silly. But I'm profoundly unconvinced that that happens very often -- even when I've seen occasional posts like those you've alluded to on MN the parents generally don't feel that they get loads of family time together normally, and that's why they are loath to give it up.