The idea of abstinence prior to marriage made perfect sense before effective birth control. Getting pregnant (or getting someone else pregnant) without a stable relationship was highly irresponsible and dangerous - socially, financially, and most of all for the child. These days it's easy to avoid pregnancy so there isn't really a need to avoid sex before marriage.
Yay. I don't think this natural assumption that sex and morality/love should be tied in together. Sex is recreation, and while it can be spiritual, like a lot of things, I don't think it should necessarily be assumed so. So long as suitable birth control is used, however. This said, I don't believe marriage is a useful institution. Or at least, I wouldn't be putting my egg in that basket. I am moved to agree with Kahlil Gibran:
"Love one another, but make not a bond of love. Let it rather be a moving sea between the shores of your souls. Fill each other's cups, but drink not from one cup. Give each other your bread, but eat not from the same loaf."
Reason: Sexual Comparability, Make sure the key fits. If not you won't be unlocking any new doors. The reason why a lot of relationships struggle after the commitment is because behind doors the consenting adults are unhappy with their love life.
I guess it depends on the couple. If it's a man and a women, I say nay because I don't think it's right for Children to have to be born out of wedlock. Of course that's an ideal interpretation of sex and marriage, but I don't suppose it's the end of the world if a couple does have sex before getting married.
One of the responses that treats the other person as a car to test drive makes sex the goal and the person an object.
The goal is to love the other person and sex is a very specific (and exclusive) means of expressing that love that should be reserved to a commitment like marriage. Man's vocation is to love, and this can be done with sex or no sex. But when it comes to a proper relationship sex should not be a 'test' or just an outlet for the couple but the cumulative expression of their friendship, fidelity, and total self-gift.
I never understood the whole sex and marriage thing going hand in hand. Skiff you love salmon, you should mako love with them. A piece of paper doesn't mako it any moor oar less alright. Sex is about love, not marriage.