I have no problem with it... but my husband and I didn't cohabitate until one week before we got married (we bought a house together and we closed before the wedding). Even then, we had to keep it a secret from his family... his mom threatened to disown him if she found out he slept with me or ever EVER spent the night with me, let alone lived with me... my mom wouldn't have liked the idea, but she wouldn't have threatened to disown me.
If it weren't for his mother, we would have lived together to save money, learn about eachother, etc... I don't think it would change anything, but I do wish I knew some of the things about him that I know how then... it would have made me think twice about marriage (and made me more sure in my decision), or I might have considered some premarital counseling that targeted our major differences (such as he's clean, I'm chaotic... and a few personality traits/issues that didn't come out until we lived together for a year or so).
All that said, I've had roommates of the opposite sex, and no hanky-panky went on.
It helped that one guy was gay and the other committed, I supposed...
Even from a religious perspective, I don't see the problem if two people are committed before God/Gods/Goddesses... who else does it matter to, anyway? The legal papers are just papers.