That's both a fact and not an answer to the question he's asking. For example, unmarried people can live together and split the bills. The above poster isn't asking if it's cheaper to live together with someone--they're asking if selection bias causes it to look that way. That is, whether or not there's sufficient distinction between single folk, long-term non-married split-the-bills relationships, and marriage.

You don't have to be married to live with someone else. That is why many people get a roommate, you split the bills and save more money.

Many people still are living together and splitting bills without being married, so it's not all that relevant to the question.