I would never be offended by any 'no children' event.
As you say, numbers can blow out ridiculously when you have children too, and that may mean not being able to invite as many people as you'd like.
Children do also change the atmosphere of an event, and some people might like a more adult environment for a baby shower.
Mine was of an evening with drinking, so it wasn't a child friendly event. I've been to others that have been very elegant high tea affairs - and for those children would have made it a different sort of thing.
I don't understand the assumption that all/ most events should cater for children too.
As long as the host understands that some people may not come (e.g., if they don't have babysitters), and is okay with that, then I don't see the problem.