My conviction would be, yes, and those who aren't legally married are living in sin (with the exception of a young engaged couple who might be shipwrecked on a desert island or something weird like that).
Notice this verse:
Matt 5: 31“It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’
32But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
Jesus is quoting from the Law regarding this subject (Deut 24:1). So, in God's economy for ancient Israel, he established legal recognition for divorce. Do you really think there wasn't a process for legal recognition of marriage?
Additionally, would a Christian, who should be intent on presenting a good witness to the world, avoid marriage?
Societal expectations have been eroded, but I think there is still some stigma upon a child if his parents are not married.
I only bring up this topic because I've heard a few older professing Christians express this idea that marriage isn't required to cohabitate and enjoy the sexual benefits of marriage. Usually they are following theologies that are very independent and separated from the normal evangelical experience. In fact, some of them are legalistic and claim that the Mosaic Law is still in effect. This is really bizarre because for all their professed attentiveness to minor aspects of the Mosaic Covenant, they are disrespectful toward the institution of marriage.