Yes, absolutely. I've never come across one that doesn't. When you flush the loo the air displaced has to go somewhere, and when you pump it out you need to let air back into the tank. Are you sure yours doesn't have one?
Or is yours one of those above-waterline jobs where you pump the crap uphill into the tank when you flush the loo, then you empty it by opening a valve at sea and all the crap just falls out by gravity? I've got a feeling they might not have normal vent pipes, but I'm not sure