Where does the Bible say that living together when not married is wrong?

 I honestly only found the answer to this question a few months ago! 
I remember talking to a friend once. She was telling me a story about a young guy from overseas who had stayed with them. He told them that when he went back home he was going to live with his girlfriend. My friend said to him, "Has anyone ever showed you from the Bible that it's wrong?" She showed him and as a result he got married instead.
When she told me that, I thought and nearly asked, "How did you show him it was wrong? Where in the Bible does it say that?"

 I definitely knew it was wrong, but I didn't know how you could clearly show someone that from the Bible.
I was too embarrassed to ask and show my ignorance.
A while ago, I was writing a devotion for my blog. I looked up the dictionary definition for the word "fornication" and there I discovered my answer!
That's exactly what fornication means--a sexual relationship outside of marriage.
(It doesn't only mean living together, because people can have that kind of relationship before or without even living together)
Living like you're married when you're not is becoming normal today, because our country is moving further and further away from the Bible. Yet not a long time ago, it was considered shocking.
But back to the question, the Bible is black and white on the issue. Although it sometimes help to understand what words mean!
Here are a few of the many verses that show us that fornication is wrong. 
Now my question is, how can any Christian possibly think that it's alright??

Ephesians 5:3 But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints;
 

1Thessaloniana 4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication:

 

1Corinthians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.

1Corinthians 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.

1Coorinthians 10:8 Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.

 
Galatians 5:19-21 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.

 Jude1:7 Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire.