Spouse of Jesus, St.Monica was not living in sin. In the early Church, dispensations were sometimes granted for such so-called mixed marriages, even between a baptized Christian and an unbaptized pagan, provided there was a necessity.
To answer MaterDominici's question, a marriage contracted between two baptized persons, including a Catholic and a Protestant, would be valid but unlawful. A marriage between a Catholic and a non-Christian however would be null and void.
So in the first case, I do not think it would count as fornication, but in the latter it would.
A dispensation for marriages of the first sort is granted when the non-Catholic person agrees to allow his/her spouse the free practice and profession of the Catholic religion, the Catholic spouse in turn promises to bring up the children in the Faith, and further to do what is possible to bring the other spouse to the profession of true Faith.