I agree with General Mung Beans and realisticidealist in principle but I recognize that many people's lives don't actually work that way and it's often not really their fault.
I think most people's lives work that way and never would have but for the veneer of a marriage as being the only way in which sexual relationships could be established. I think marrying prior to establishing sexual compatibility or even establishing a personal space can be quite dangerous.
I definitely agree as far as the personal space side of things goes. I'd say it's a downright good idea to at least spend some time feeling out the basic elements of space-sharing and going about daily life together before getting married and perhaps before deciding to, and a bad idea not to. I don't feel that way about sex but, well, I wouldn't know, really.