Nobody SHOULD have to explain anything. So it's okay to not do it. but if you're on a date, where you're trying to put yourself out there and establish at least a friendly relationship, you probably want to be cordial and share your thoughts on the matter. Instead of just shutting down mid-date.

I don't consider it a defense, exactly. It's more clarification. Just saying "no kids" might suggest he doesn't want kids ever, which would reduce the potential partners unnecessarily (and if he does want kids eventually, being paired with someone specifically because they don't want kids would just create problems later). Saying "no kids yet" sets them up with someone who doesn't have kids but might in the future.