Last year when I filed taxes the lady that did them at hand r block told us that if we filed together we would be legally married. Is this true?

It depends on where you live. In Texas, it would possibly be true, if you meet the following conditions:

What makes a common law marriage?
A: Three elements must be present to form a common law marriage in Texas.

First, you must have "agreed to be married."
Second, you must have "held yourselves out" as husband and wife. You must have represented to others that you were married to each other. As an example of this, you may have introduced you partner socially as "my husband," or you may have filed a joint income tax return.
Third, you must have lived together in this state as husband and wife.

(Common Law Marriage, Travis County, Texas)

Other states don't recognize common-law marriages, in which case you may be committing tax fraud if you file as married when you aren't. Not sure about that.