Different but equal is not equal. Surely that much is obvious given history.
Marriage is a social construct (and most certainly not a religious invention), and has evolved over the years many times, so there is no one true definition of it. One part is love and companionship, the other is contract and social/family ties, and so on - it varies a lot between societies and within societies/individuals. But to say to one group in society that they must have something different, even with similar legal protection, is saying they are not part of that society. That is unequal and unjust.