I think it's more that some people see their DCs as part of their unit, so are offended in the same way they would be offended if the invite was just to them, not them and their DH/W/P, even if their DP doesn't know the couple.
And other people do see weddings as more of a community event, not an event about the couple (like Startail) - unfortunately, this view might have been true when couples predominately married when still young, married people in their community with extended families who, if not knowing each other beforehand, would come in contact again, and their parents footing the bill and effectively was a party thrown by the extended family for the couple. However, it's very very rare to find a wedding like that anymore. Of the 45-50 of our wedding guests who weren't family, my parents knew 3 (uni housemates and old friend from home town). While our two sets of parents had met, this was the first time my parents had met DH's Aunts, Uncles and cousins, it was the first time PIL had met mine, in the nearly 6 years we've been married, aunts/uncles from both sides have seen each other once more at DS's baptism.
We footed the bulk of our wedding bill - it was a party that set us back £20k, I think it's rather disgusting to happily let someone spend that much on a party and then get an arse on because they'd like it to be about what they want and be focussed on them. Weddings are one of the few events where other people feel they have 'ownership' of an event they aren't hosting/paying for.
We had a childfree wedding by accident, at the time only one couple we invited had DCs, (who were older than the sort of toddlers you'd worry about containing for the vows) but they couldn't make it, so we didn't need to think about it. (I suppose my cousin counted still as a child at 15, but she also was perfectly capable of not causing any problems). However if we'd got married now, not counting our own DS, the same guest list would include 18 DCs, at least 15 of those would require a seat and a meal, and I seem to recall it about about £30 a head for DCs - so an extra £450 at least (plus whatever it cost in increased venue hire if we had to move to the larger room at the hotel). We could have found that (although we'd probably have had to change the timings, and I doubt if we had that many people with DCs to take home/go back to the hotel room to relieve the babysitters, we'd have had anywhere near the £7k bar bill in the evening so it might have evened out), but a lot of people can't afford that.