I've had friends and family get married that were living apart and living together prior to marriage. I am happy for both types but I get more excited for the couples who do not share living space. Primarily because the decision is more "momentous." They do not share a household, finances, plumbing issues, bills, illnesses (puking the bed at 3 am anyone?) and all the drudgery that life entails, yet. It is a complete change of lifestyle as the previously single man and woman know it. That's why there's a lot of winking, nudging and snark at the bride's shower, a mournful dirge at the groom's party, riotous behavior, etc. Then the couple celebrate together and married life begins.
With couples already living together, they have their household established, maybe a couple of kids, there's a nice party and life goes on as before. There's no real life change (as many have said they were already "committed" beforehand).
So maybe grandma is old-fashioned (I must be too), but I don't see her as malicious, just as having a "been there done that" why all the fuss attitude. I hope she can be happy for the couple, but to be mad because she can't force "excitement" is looking for offense.