1. It makes it too easy to drift into marriage.
2. It makes the proposal anti-climactic.
3. It renders most wedding traditions meaningless.
4. It sends the message the marriage is not important to you.
5. It limits your options.
The most important of these, by far, is the first:
It makes it too easy to drift into marriage
Practical problems like financial pressures or roommate issues can make moving in with your boyfriend or girlfriend seem to be the easiest solution, whether or not you're certain that this is the person you want to be with for the rest of your life. Then, as the months turn into years and you're still under the same roof, you naturally start thinking about marriage -- if nothing else because it seems to be the next logical step. If you've been living together long enough and things are going fine, eventually there's a subtle pressure that makes it seem like having a wedding is something youdo. And when you haven't had the space (literally) to take a step back and objectively consider whether this person is truly the best match for you, the situation is ripe for sliding into marriage by default, rather than getting married as an active, conscious choice that you're genuinely thrilled about.
Read more: http://www.ncregister.com/blog/jennifer-fulwiler/5-secular-reasons-not-to-live-together-before-marriage#ixzz2L4XkXA3t
I cannot emphasize on this point enough. If you are already having sex with someone, and living together on top of it, the next natural step is to make it official. Having sex with someone releases oxytocin which naturally creates a bond between the two partners.
Recent studies have begun to investigate oxytocin's role in various behaviors, including orgasm, social recognition, pair bonding, anxiety, and maternal behaviors. For this reason, it is sometimes referred to as the "love hormone". (http://en.wikipedia.org/wiki/Oxytocin)This works in much the same way that breastfeeding greatly promotes the bonding between mother and child. It's the same hormone.
These days, most people seem to be trying to ignore this psychological bond, by constant "casual sex" but the bond is still there, however much ignored. And it makes it ALL THE MORE HARD to leave a relationship that you know is not the right one for you, if you have already had sex with the person. It is so much easier to break it off with someone you have never had sex with. The heartbreak is much, much less, and there is no regret. You may even decide to remain friends, because there is no complicated history.
This is without mentioning that focusing on WHO the person you are with is, instead of focusing on how they make you feel and how much pleasure you can get with them is EVER SO MUCH BETTER for creating long-lasting and HAPPY relationships. This last point is extremely important. Sex is fleeting. Sexual attraction does not last forever. A relationship based on sex will not last, or be a happy one.