Living together before marriage?

My boyfriend and I have only been together for about two months but we're talking about getting a place together. Not as "the next step" in the relationship or anything, but because we have two mutual friends who really want to move out of their parents houses too but need two more room mates to pay rent with.

Whenever I tell people I'm planning to move with my bf they seem to like, flip out on the inside, but aren't able to express it. It's kind of amusing to watch really.

Believe me I get the whole, 'don't make your lives so ingrained that it would be difficult to move on if things go wrong' dealio, but plenty of fucking horrible things have gone wrong in my life, so I think I could deal with it just fine... and we already practically live together, I mean we spend 24-7 together. Like he goes home every three days to change clothes and then comes back.

Anyways, what's the big deal of a couple moving in together before marriage? I thought it was a very necessary step. What did you do/plan on doing?