The following question came to me via Formspring:
where, in the Bible, does it say that a couple cannot live together before marriage? Or, does it?
Here's my answer:
I don't know of a place in the Bible that says that a man and woman cannot live in the same dwelling if they are not married. There may be some Old Testament rules/customs related to that, but I don't see a moral reason why that wouldn't be OK. Now, if you are using "live together" as a euphemism for "have a sexual relationship," that changes the picture a bit.
I can think of two reasons off the top of my head that sex outside of a marriage relationship is not the way of believers in the Bible. One reason is that the Bible is pretty clear that two individuals who join themselves in a sexual relationship "become one flesh" (see 1 Corinthians 6:16 http://esv.to/1Cor6:16). Once you have united yourself to someone else, you can't un-unite... in some ways it would be like an amputation. That's why divorce is so emotionally horrible; it's severing one flesh. So having sex with someone and not being married to them is leaving yourself open to an emotional crash later on; with the marriage commitment in place there is a higher barrier to walking out the door. The barrier is for our own protection.
The other reason is that marriage is a picture of Christ's relationship to His bride, which is the Church (consisting of all Christians, not any specific group but all believers in Jesus) (see Ephesians 5:22-33 http://esv.to/Eph5:22-33). Doing an end-run around marriage in favor of having a sexual relationship while unmarried, even a stable, long-term relationship with commitment, paints a twisted picture of Christ's love for us. Jesus loved us enough to make a commitment, and a man should love his woman enough to make a commitment to her as well.
You could find a lot more reasons and arguments out there on the Web, but those are the ones that spring to mind. You might also take a look at these lists of Scriptures about Marriage and Sex on my site.
Things the Bible says about marriage:
Things the Bible says about sex:
More of my Formspring answers