I was just having a conversation with my mother.... I moved in with my SO a few months ago and she said "well I guess you guys are pretty serious." She comes from the school of not-living-together-until-marriage. I said that I would never marry someone unless I had lived with them for a significant amount of time first. She nodded and said something along the lines of, "Yeah it's different from when I got married, but not a bad idea." The idea of marrying someone when you've never even been alone in a room with them..... I just can't comprehend it. It's absolutely mind-boggling.
I've heard people (mainly nosy aunts) say things along the lines of, "if you live with him before marriage, he'll drag his feet and never propose," and "if you live together before marriage, what changes afterwards? What is there to look forward to?" Ummmmmmm a lifetime of love and commitment with a totally awesome human? That's pretty cool. If you're marrying someone mostly because you want to live with them/have sex with them, that's a pretty poor reason to get married, in my opinion. I live with my SO because I want to, and I love seeing his face every day. If I ever want to marry him, I'll marry him, because I want to spend my life with him. Simple as that.
Even if you do live apart/save sex for marriage, those decisions should be really small blips in the grand scheme of spending your life with someone...... the person is more important than the penis. You can quote me on that.