Marriage penalty relief is likely to create some circumstances in which at equal total income levels two singles will pay higher taxes than a couple, or one couple will pay higher taxes than another couple. And, of course, there are economies of scale in living together. The plain fact is that there are all sorts of living arrangements today. A very large percentage of single individuals live with others. Cohabitating couples are only the tip of the iceberg.
People live together in dormitories and nursing homes.
Friends and relatives constantly share living quarters. Often young adults return home and live with parents for awhile. Living together and sharing resources is a natural condition of the human species. If one should tax married couples more because of the advantages they derive from living together, then one should also increase taxes on nuns living in convents together and soldiers sharing barracks. Flip the argument the other way.