I'm not going to argue my objection from a fornication standpoint, because that's not an issue I really care about.
I do think, however, that living together with someone before marriage is a bad idea if you are already on the road to getting married. I think it might cause you to end the relationship prematurely when it could in fact actually be one worth saving.
Don't get me wrong. I'm not one of the "destiny, God has brought us together, lets fight for this relationship when it's actually a turd relationship" type of people. Some relationships are mistakes and the sooner you abandon them the better. But, there is always going to be a normal level of conflict in any living-together type of situation. You are not 100% compatible with anyone - and things that they do (and that you do) are going to get on each other's nerves.
Most of it is superficial...but annoying. Even in great relationships, that dynamic exists. My fear would be that if I didn't have a little bit of extra incentive to work through those annoyances - that it might become catastrophic to what otherwise is a good relationship that ought remain.
Don't take that to mean that you should work through anything that crosses your path...lol But, I've seen people also break up over what to me seemed like petty easily-rectified stuff. Maybe if they'd been married they would have tried a little harder.
Just my .02