Here is some scripture...
Genesis 2:24-25
"For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh. And the man and his wife were both naked and were not ashamed."
This talks of two people coming together in marriage not as 'room mates.' If there is nakedness as 'room mates' there will be shame or lust and other impure emotions. As husband and wife there will not be shame but love and pure emotions.
Romans 1:24
"Therefore God gave them over in the lusts of their hearts to impurity, so that their bodies would be dishonored among them."
This passage talks about God giving people over to their lusts and desires. In the previous passages it mentions that this applies to those who know God and what He says in His Word. It is a grave thing to know what God commands of us then not follow it. It also talks in the previous verses that those who try to be wise in fact make themselves fools because they go against what God has written in His Word.
Romans 12:2
"And do not be conformed to this world, but be transformed by the renewing of your mind, so that you may prove what the will of God is, that which is good and acceptable and perfect."
We are not here to be as others are, we are to be different. We are to be transformed by Gods Word and that that transformation will show what is good and what is not good. I think that if you spent time reading Gods Word you will find your answer to this.
2 Timothy 2:22
"Now flee from youthful lusts and pursue righteousness, faith, love and peace, with those who call on the Lord from a pure heart."
This is pretty straight forward. It says flee. Run! Get away. It does not say get as close as possible as long as you are strong enough. It says flee! It does not say to get close as possible but do not sin. It say flee! It does not say get as close as possible as long as it is convenent and it is helpful for you finaically. It says flee!
Too many people think that they can withstand temptation but in the end fall in to it. Even if you do not fall in to it think about the example you are giving to others. Think about your brother or sister. Or think about your children. What happens if they are not strong enough and fail. Your example is saying that what you are doing is ok and there is nothing wrong with it - whether you say it or not. Do you want to be responisble for helping your brother or sister to sin?
Remember that what you do speaks much louder than anything you say.
Although there is no passage in scripture that specifically says that you are not to live together before you get married, it is very easy to see that by many other passages that doing so is wrong. What you are doing is putting your self in a place where sin is just ready and waiting to pouce on you.
Why do you think that there are so many Christians that are against this sort of thing?
There are so many verses that speak against immorality that it would take pages to list them. These are only but a few but here are some references to them:
Romans 13:13
1 Corinthians 5:9, 11
1 Corinthians 6:9, 13, 18
Galatians 5:19
Ephesians 5:3
Colosians 3:5
1 Thesselonians 4:3
Jude 7
God bless and please flee from all evil not just lust.