I recently read somewhere about elderly couples living together as a married couple, without the benefit of actually being married. The article suggested that they stayed unmarried because of "dire tax consequences", if they actually tied the knot.
In at least most cases, the writer had it wrong, usually it is advantageous to be married as far as taxes are concerned. I believe it is even possible in some jurisdictions to file joint returns when the folk involved are members of a domestic partnership or registered civil union.
No, when people say "they can't get married", they usually mean that they have chosen not to get married, because of financial considerations. Usually, this is because one of them is receiving a "widow's pension". I am certainly not going to sit in judgment over those who make that choice, in the light of current practice. However, I do think we should look at what is behind the reluctance to get married. In times past, the man was usually the principal, if not the only, breadwinner. The expectation of life of adult males was shorter than that of females. A considerate society felt it was appropriate to provide some support for impecunious widows. However, many widows remarried, and of course they no longer needed a widow's pension. So the employer, trade union, or even the state, could save the money.
I consider this a very dated concept. Men and women are both eligible for their own Social Security pensions. Admittedly, many of those women now receiving Social Security benefits spent years as homemakers, or worked in low-paying jobs; as a consequence, their benefits are often relatively small. But "times they are a-changing", and I believe that any pensions paid to women (or to men!) after the death of a spouse should continue until the survivor's death, whether or not the survivor remarries. Also, the importance of pensions for survivors has diminished, and in the present state of the economy, we may find that they are gradually abolished by most employers.