"Unless their parents aren't sensible enough to take them out of the church if they start crying/running around or whatever?"
That has been my experience, more often than I have encountered sensible parents who do take their children out / stop them running around during the speeches / Stop them getting up and down from the table between each sitting and so on...
... But that's no fun for the children though is it?
That for me is the crux of the matter - I had to take my DD to a wedding last summer (no-one to babysit on this particular occassion), and I felt that to keep my DDs behaving appropriately, I had to bribe, shout, moan, cajoule - you name it to get them to behave (and yes, I did take a big bag of brand new toys for them!) They found the service boring, the photos boring, the meal boring (and they didn't like the food), the speeches boring and there was nowhere for them really to run about, be noisy and just let off steam. For my kids to enjoy themselves, it involves running around, being noisy, doing what 4 and 1 year olds do - but that is not conducive to appropriate behaviour at a wedding, and that is why the two essentially do not mix - Its not being miserable, its just sensible! (Although I do accept that this is different for babies and older children!)