Take the other alternative for a moment. If you don't live together first, you may be in for some very unpleasant suprizes if you DO decide to get married later. Don't you owe it to yourself to see what the other person is all about? What if you didn't experience the other person first and got married later, and then found out that this person had some serious sexual or personality problems. You would be stuck in a relationship that wouldn't work at all.
If you decide to buy an expensive car or buy a nice piece of clothing, you would try it on first and see how it 'suits' you. I feel the same about deciding to live with some one. If in the long run things don't work out, there is no harm done. Divorce is painful and ugly. So it's perfectly normal, and wise to live with someone first.
My three cents