Why not just look at the dictionary.com definitions of "sacred"? One of them is indeed objective or exclusive: "entitled to veneration or religious respect," which would account for the religious conflicts we see in history, since each religion might deny that what other religions regard as sacred is actually "entitled" to be worshipped.

But then another definition is more subjective: "devoted or dedicated to a deity or to some religious purpose." So something becomes sacred just because it's devoted to that purpose. Or another: "reverently dedicated to some person, purpose, or object." Or another: "regarded with reverence."

But you go ahead and stick with your definition. This disagreement is purely semantic, not philosophically interesting.