How nice, Sharingan, to refer to those who oppose this as "primitive".
I don't think it is primitive at all to do so. This may be the right change for Society as a whole. But Society has been brought up, perhaps wrongly, with an idea of marriage as a particular type of bond. Attitudes rooted so deeply aren't going to change overnight. Redefining marriage isn't just about scribbling over the old definition in a dictionary either, and there will be inevitable legal consequences. In turn, these do affect how current marriages are seen under the law -- in laws surrounding custody in divorce, for example, "mother" and "father" will likely have to be changed to "parents one and two". So legal recognition of well-established roles changes.
The change will perhaps, in 50 years' time, not make anyone beat an eyelid. But here and now, it does matter, and that should be respected and not brushed aside as either bigoted or primitive.