It depends on who you ask. But for me it is imperative that two people live together before making any commitments that will effect the rest of their lives.
One does not buy a pair of shoes, or pair of pants without trying them on first. Why should a bigger commitment not deserve the same?
I have known many couples who made marriage commitments before knowing if they were sexually compatible. In some instances they adapted, or suffered in silence if they were not compatible. Most people who find themselves in a relationship that is not mutually compatible will end in separation, and rightfully so.
The old laws of morality have (or should have by now) fallen along the wayside when it comes to relations outside of marriage.
Also, those who think that two people living together without marriage is immoral have no business judging others, so their thoughts or opinions should be of little, or no, consequence anyway.