Our society has become more and more secular in recent years, and I’m afraid that what your son has done has become increasingly common—so much so that many people today see little wrong with it.
But the Bible is clear: sexual relations outside the covenant of marriage are wrong in God’s eyes—and for good reason. God loves us, and He knows that any relationship like this is seldom secure and lasting. The reason is because it lacks the one thing that comes with marriage: a vow of commitment—both before others and before God. This is why the Bible says, “Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral” (Hebrews 13:4).
Right now, however, you face a practical issue with your son, who knows your views and yet has chosen another path. He alone is responsible for his actions—but you also have the right to oversee the use of your house. Let him know your concern (although without arguing); you might offer to find space for her with a neighbor or family friend.
Above all, ask God for wisdom in your relationship with your son. Don’t be harsh or judgmental. Someday his girlfriend may become his wife and the mother of your grandchildren. Most important, pray that he may realize his need for Christ and become His follower.