Funny thing-- it's not too much of a stretch to extend this to pet ownership, as well. I have mixed feelings about all of it, honestly, but since these are essentially intractable problems at this point in time, I believe it comes down to how each individual animal is treated.
I certainly think it's quite possible to have a large, 'exotic' animal who is trained to perform from birth to be happy and have a decent quality of life, just as I think the same is possible for that puppy or kitty adopted by someone you know. It seems clear to me that neither is ideal, but there are plenty of examples of better and worse situations. It's not black & white.
Regardless, in my view, anyone concerned enough to protest the treatment of animals in circuses and/or zoos is being hypocritical if they're not working just as hard (or harder) to end the suffering of all the other sentient creatures we subject to 'unnatural' conditions on a vastly larger scale. Right now, the best we can seem do is keep saying "spay and neuter" like we have for at least half a century, and too many people still don't even seem to be getting that utterly basic message.