Why is marriage referred to as 'tying the knot'?

your right, in pagan days (and still now sometimes) it was called a hand fasting, it originates from Scotland, and the couple's hands would be tied together in a ribbon, for a day and night, hence the saying "tying the knot" couples would still exchange rings, and traditionally the term would be for 18 month, but most couples would opt for the longer term, nowadays it is still somewhat popular but it is not legally binding.

http://www.mrs2be.com/handfasting-pagan-wedding.ph...

http://www.handfasting.info/

Edit.. it has nothing to do with the Romans

Tying the knot

This comes from the days of the Roman empire when the Bride wore a girdle that was tied in knots. The Groom untied the knots prior to the consummation of their marriage.

YES PEOPLE WERE TIED TOGETHER DURING THE TRADITIONAL ANCIENT PAGAN CEREMONIES WITH REEDS MADE OF VARIOUS MATERIALS ONCE THEY FINISHED THE CEREMONY, THE OFFICIATOR HELD THE REEDS DURING THE VOWS, AND ONCE THE COUPLE FINISHED THE VOWS, THEY WERE TIED TOGETHER FOR THE REST OF THE DAY IN CELEBRATION OF THEIR UNION.

They still are tied together - albeit very loosly with the vicar/priests sash.

The knot in the noose round your neck.

The expression came into use in the 1200's, and is symbolic of the "bonds of matrimony."