Yes, up until the point of paying it is theft
You have eaten something without paying what else would it be called
A restaurant? Or do you settle up in advance? Or if we're talking supermarkets, then "a Waitrose free coffee". 
To be theft, you must intend to permanently deprive - i.e., not pay. If you intend to pay, then it isn't theft. That's, you know. The law and all. It's not theft in any legal sense whatsoever, and as there's no consensus at all on it being wrong, you're on a sticky wicket calling it morally so, either.
Funnily enough, as this comes up really often on MN I asked our local Waitrose. The staff unanimously said that if they had to choose between a baby or toddler screaming their way around the store, or a happy child boosting profits by eating something Mum paid for later, then they would opt for the second, every time. I never asked about adults, because frankly it never crossed my mind. For no reason I can really voice, I agree it's a bit naff for adults to do it. You can wait five minutes, surely? I don't have any logical argument, but I don't find it very couth. It's the sort of thing my Granny would have been appalled by.
And people who eat and don't pay are stealing, yes.