Nah. People living together. The Bible describes adultery as "sex outside a marriage" (See the woman at the well).
That is correct, of course, but in modern parlayance, I think "fornicator" would be the more common term.
People who participate in such behavior should be welcome in the church so that they might hear the truth and turn away from sin. Lowering the bar set by Him to accomodate such behavior means the parish is preaching a false gospel.