Visiting Cohabiting Son at His Home

Your approach is a morally sound one. You always want to maintain communication with a loved one who has strayed, as Jesus never abandons a lost sheep (Luke 15:1-7). And so you welcome your son and his cohabiting girlfriend to visit you in your home. At the same time, you don’t want to compromise truth by your actions, and so you convey a charitable message that because they are not committed in marriage, despite being sexually intimate with one another, you cannot allow them to spend the night together in your home, unless in limited circumstances in which they sleep in separate rooms.

Similarly, there is no moral problem with your visiting them in their home. The same rule would apply. Just don’t spend the night with them, given that they are attempting to live as husband and wife without being in a committed lifelong marriage (see Gen. 2:3-24).

For information read this short answer on why cohabitation is morally wrong.