Question:
Answer:
For the purpose of this answer, I’ll assume by “significant other” you are referring to a romantic partner of the opposite sex to whom you are not married and who is not married to anyone else. (If any of these conditions are not accurate, then the definition of the sexual sin in question changes and may increase in gravity.)
Cohabitation between unmarried partners of the opposite sex is an occasion of two sins, not just one. The first is, of course, fornication (sexual relations outside of marriage by an opposite-sex couple). If sexual relations are not occurring during cohabitation, then the couple is not guilty of fornication. They may, however, be placing themselves in a near occasion of sin—which means proximity to the temptation of sin—and we are supposed to avoid near occasions of sin.
The other sin is scandal, which is defined by the Catechism of the Catholic Church:
Scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense (CCC 2284).
A couple who lives together outside of marriage creates scandal by engaging in an action that may lead others into sin, either by allowing people to believe that they are engaging in fornication or by tempting people to believe that cohabitation outside of marriage is okay so long as the couple does not engage in sex. Either way, the couple is creating spiritual obstacles for others, and to do so is indeed an occasion of sin.